{'fraction_non_alphanumeric': 0.03937130407718643, 'fraction_numerical': 0.018269530034235915, 'mean_word_length': 5.083593676039004, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
LATE CRETACEOUS OYSTERS FROM THE PACIFIC SLOPE OF NORTH AMERICA : REVISION OF NAMED SPECIES AND DISCOVERY OF NEW SPECIES 1

Late Cretaceous oyster species are common faunal components in shallow-marine siliciclastic rocks on the Pacific slope of North America (PSNA), in a region extending southward from Vancouver Island, British Columbia, Canada, to Northern Baja California, Mexico. Seven species have been named, but they are poorly known. The purpose of this study is to better document these species by upgrading their morphologic, taxonomic, geographic, and stratigraphic data. In addition to the seven revised species, four new species, a questionable species, and an unnamed species were detected, resulting in a total of 13: Oscillopha popenoei sp. nov.; Rastellum sp., aff. R. macropterum (J.D.C. Sowerby, 1825); Curvostrea crescentica (Packard, 1922); Curvostrea baia sp. nov.; Acutostrea taxidonta (Packard, 1922); Crassostrea brewerii (Gabb, 1864); Costagyra californica (Packard, 1922); Costagyra garza sp. nov.; Amphidonte parasitica (Gabb, 1864) [assignment to this genus by Elder (1991) is confirmed]; ?Amphidonte parasitica (Gabb, 1864); Pycnodonte (Pycnodonte) malleiformis (Gabb, 1864); Phygraea inornata (Packard, 1922); and Phygraea arida sp. nov. Morphs most likely represent ecophenotypic responses to different water energies (e.g., A. taxidonta) or whether or not there was attachment (e.g., P. inornata). Growth stages were newly recognized for A. parasitica. None of the detected oyster genera or subgenera are endemic to the warm-temperate PSNA region. Except for Amphidonte, none were previously recognized in the Cretaceous record of the PSNA region. The late Santonian Oscillopha popenoei is the first occurrence of this genus in North America. The highest biodiversity level of PSNA oyster species (five) coincided with the Turonian and late early to early late Maastrichtian (both warm times with high sea level), and the lowest biodiversity level (zero) coincided with the Coniacian (cooler time with lower sea level). In the PSNA region, there was a reoccurrence of the Turonian Curvostrea-Costagyra-Phygraea association in the late early to early late Maastrichtian.


INTRODUCTION
Oysters are common in Upper Cretaceous shallow-marine strata in the region extending from Vancouver Island, British Columbia, Canada, to Northern Baja California, Mexico (Fig. 1), referred to hereafter as PSNA (Pacific Slope of North America). Seven species have been named. Gabb (1864) named three, based on collecting in Northern California, and Packard (1922) named four, based on collecting in Southern California. Their descriptions are inadequate because they are too brief and rely on only the holotype. Their illustrations are commonly insufficient because they show only one view of a badly weathered holotype or show only its interior. In addition, Gabb's illustrations are idealized sketches. Subsequent workers encountered difficulty in trying to recognize these species and generally avoided using Gabb's and Packard's names, as evidenced by the short synonymies given here.
The purpose of this study is to improve on the fundamental paleontologic information concerning these previously named species. The improvements include 1) more detailed morphologic descriptions, including recognition of morphs and xenomorphism, 2) all the photographs necessary to document the observed morphologies, 3) integration of current concepts of familial classification of oysters, 4) integration of more comprehensive geographic and stratigraphic distributions, and 5) updated chronostratigraphic data. During the course of this study, four new species, one unnamed species, and one questionable species were also discovered. The revisions and these new discoveries have allowed, for the first time, meaningful biodiversity information concerning the oyster fauna to be inferred. The significance of this contribution is that it establishes a modern foundation for future studies of Late Cretaceous PSNA oysters.

MATERIALS AND METHODS
The collection at the Natural History Museum of Los Angeles County, Invertebrate Paleontology (LACMIP) Department, is the principal source of material. Two of the core strengths of the collection's Cretaceous holdings are 1) Turonian through Campanian fossils collected by W.P. Popenoe and L.R. Saul in the Santa Ana Mountains, Orange County, California, and 2) Turonian fossils collected by W.P. Popenoe in the Redding area, Shasta County, California. The specimens they collected are critical for the revisions reported here. More recent additions of well preserved fossils to the LACMIP collection were essential in confirming and expanding the work of Elder (1991) on Amphidonte parasitica (Gabb, 1864).
Specimens from British Columbia, Canada, were the sources for supplemental material. Although Cretaceous strata are plentiful along the southeast side of Vancouver Island, British Columbia, oysters are not common in these strata, except at a few localities, like near Nanaimo Beard, 1994, 1997) and on western Denman Island (R. Graham, personal communication, 2015). Raymond Graham facilitated the loan of some Nanaimo material collected by Graham Beard and now part of the Vancouver Island Paleontological Museum (VIPM) collection in Qualicum Beach, British Columbia. Raymond Graham photographed these specimens and provided detailed stratigraphic information. Raymond Graham also lent some western Denman Island specimens collected by him and Timon Bullard. Two of R. Graham's specimens and one of T. Bullard's specimens are used here as paratypes of Phygraea arida sp. nov. These three specimens are now part of the Royal British Columbia Museum (RBCM) collection in Victoria, British Columbia, Canada.
Upper Cenomanian oysters occur predominantly as internal casts, whereas Turonian through upper Maastrichtian oysters are generally The suprageneric classification of oysters has been in a state of flux during the last two decades. The most current classification scheme, and the one used here, is from Carter et al. (2011). Their scheme will be used for the pending revision of the ''Treatise on Invertebrate Paleontology'' volume on oysters. Standard morphologic terms are from Stenzel (1971), and those most commonly used here are illustrated in Figures 3-15. Shell-microstructure terms are chiefly from Malchus (1990) and Jaitly et al. (2014).
In compiling the species synonymies, references with accompanying illustrations were used almost exclusively. Nonillustrated references, such as a checklist, were used, but only if museum specimens of the species referred to were seen, thereby allowing for confirmation that the species occurs in beds relatable to the checklist.
To facilitate comparisons, photographs of each studied species were arranged in the following manner: left valve exterior; left valve interior; right valve exterior; and right valve interior. In some cases, anterior or posterior views of a single valve or of conjoined valves are shown, as well as the shell microstructure.
Information about the LACMIP localities is accessible through the website link http://ip.nhm.org/ipdatabase/locality_show. Information about the type localities of the Museum of Paleontology, Berkeley (UCMP), is accessible through the website link http://ucmpdb.berkeley. edu. Information about the Canadian localities is incorporated into the appropriate ''Stratigraphic Distribution'' sections. Order Ostreida Férussac, 1822 Superfamily Ostreoidea Rafinesque, 1815 REMARKS. Fossil oysters are easily recognizable by their distinctive ligamental pit (resilifer), large adductor muscle scar, and, on certain species, well-developed chomata (denticulations on margins of the valves). Fossil oysters can be abundant, commonly well preserved, and their medium to large size allows for easy detection of the group, either in the field or in museum collections. Generic and specific identification of oysters, on the other hand, can be challenging. As mentioned by Koppa (2015:4-5), the main reasons for this are that they have relatively few unique characters, they likely exhibit significant morphologic variation due to their well-known ecophenotypic plasticity, and they display a distinct propensity for convergence. Malchus (1990) revised the classification of Mesozoic oysters considerably and restricted Ostrea Linnaeus, 1758, andLopha Röding, 1798, to the ''Tertiary,'' although these genera had been previously regarded as also occurring in older strata. Malchus' revisions are incorporated here.

SYSTEMATICS
Oysters range from Late Triassic to Holocene (Stenzel, 1971:N1051), but they did not become common faunal elements until the Jurassic. Depending on the species, they are indicative of shallow-marine or brackish-marine environments.
AGE AND DISTRIBUTION. Late Cenomanian to Maastrichtian and widespread (Malchus, 1990;Dhondt and Jaillard, 2005). The new species named below is the first record of Oscillopha in North America.
DESCRIPTION. Shell size up to height 124 mm (incomplete). Shell oval, rectangular, or slightly crescent shape. Both valves wedge-shaped, with steep plicate wall on anterior margin and much lower plicate wall on posterior margin. Hinge wide, flat, with flat bourrelets and resilifer indistinguishable. Adductor muscle scar large, ovate, rimmed (especially on anterior side), posterodorsal, and not close to valve margin. Shell microstructure foliated, with lens-shaped layers. Left valve: Lowly convex with medium to large attachment area covering up to approximately 75% of valve. Plicae 20-25 in number (two per 7 mm, crest to crest), moderately dichotomous, strong, chevron-shaped, with slightly rounded tops. REMARKS. Five specimens were examined: three left valves and two right valves. Although there are no complete valves, preservation is good, except for the hinge of the right valve. The hinge is missing on all the left valves.
The sculpture on the new species, especially on the paratype (Fig. 25), is similar to an unnamed variety of Actinostreon dichotoma (Bayle, 1849:365-366, pl. 18, figs. 17-18) from Tunisia, north Africa. Pervinquière (1912:pl. 14, fig. 21) illustrated this variety, which is of Coniacian age. Douvillé (1904:274-275, pl. 37, figs. 1a, 1b) figured A. dichotoma of Senonian age from Iran. The Redding Formation species differs from all these illustrations of A. dichotoma by having an obvious attachment scar and more closely spaced radial ribs, which are less dichotomous. The Tunisian and Iranian specimens have a posterior auricle that can be small to prominent, but it cannot be determined whether or not a posterior auricle was present on the Redding Formation specimens.
The only other record of Oscillopha in the Western Hemisphere is Oscillopha syphax (Coquand, 1862), which Dhondt and Jaillard (2005:pl. 1, figs. 6a, 6b) reported from upper Cenomanian strata in northern Peru. The new species differs from this Peruvian species by having half as many radial ribs.
ETYMOLOGY. The new species is named for molluscan paleontologist Willis Parkison (''Parky'') Popenoe, who collected and donated many Late Cretaceous oyster specimens to the University of California Los Angeles invertebrate-fossil collection (now housed at LACMIP) and consistently provided detailed and reliable locality data.
AGE AND DISTRIBUTION. Late Middle Jurassic (Callovian) to Maastrichtian and widespread (except for polar seas) (Stenzel, 1971). The unnamed species described below is the first record of Rastellum in the PSNA region.
Rastellum sp., aff. Rastellum macropterum (J.D.C. Sowerby, 1825) Figures 9, 26-32 REMARKS. Three specimens were examined, and they are from the middle to upper Turonian Frazier Siltstone Member of the Redding Formation, east of Redding, Shasta County, Northern California [Area 5]. Two of the specimens are from LACMIP loc. 10745 and occur in the same hand specimen. One is a nearly complete left valve (height 75.2 mm) , and the other is a fragment of a left valve (Fig. 32) attached to the larger specimen. The nearly complete left valve, which only shows its very concave interior, is strongly curved (crescentic), very arched with steep parallel flanks, and has a posterior auricle. Numerous angular and prominent radial plicae of uniform size are present along the valve margin and form a zigzag commissure. There are two plicae per centimeter. Most of the hinge of this valve is missing, but along its weathered anterior area there is much foliation of the shell layers. The adductor muscle scar is large, dorsoposterior, and very close to the posterior margin of the valve. The scar is filled with rock matrix and weathers quite differently from the rest of the shell (Fig. 27). On the partial left valve in the hand specimen (Fig. 32), the weathered hinge area consists of foliated shell with no distinct ligamental resilifer/bourrelets preserved, and the adductor muscle scar is crescent-shaped. There are no catachomata. The shell microstructure is foliated/homogeneous.
The third specimen of Rastellum sp. is a fragment of the interior of a dorsal surface of a valve from LACMIP loc. 24336 (Fig. 31). It shows the characteristic dorsal keel, as well as remnants of strong plicae.
The three specimens probably represent a new species of Rastellum, but it is not named because the specimens are so few and incomplete. They show the most affinity to Rastellum macropterum (J.D.C. Sowerby, 1825:105-106, pl. 68, figs. 2, 3), on the basis of the large posterior auricle and on shell size. Stenzel (1971:figs. J138, 1a, 1b) figured Sowerby's species, which is of Maastrichtian age from the Netherlands. The hinge of the PSNA species, however, is more similar to the hinge illustrated by Cooper (2002: fig. 1A) of the southeastern African Rastellum allobrogensis (Pictet and Roux, 1853:524-525, pl. 49, figs. 1a-1c) of Albian age.
AGE AND DISTRIBUTION. Early Cenomanian to Maastrichtian and widespread (Seeling and Bengtson, 1999). The species revised here and the new species named below are the first reports of Curvostrea in the PSNA region.
Curvostrea was an epifaunal oyster whose cemented left valve is larger and thicker and has a higher preservation potential than its much thinner (lid-like) right valve, which was easily fragmented. This warmwater oyster lived in well-oxygenated, normal-marine conditions associated with a moderately soft substrate and preferred low to intermediate energy levels, with a low degree of environmental stress (Ayoub- Hannaa and Fürsich, 2012:111, 125). Its shell microstructure is prismatic/foliated (Malchus, 1990;Jaitly et al., 2014).
REMARKS. Thirty-two specimens were examined, and many are fragments. Preservation is poor, and exfoliation is common. The holotype, a right valve, has a worn exterior (Fig. 40). All the specimens are in siltstone. Six specimens at LACMIP loc. 8195 in the Baker Canyon Member are conjoined. At this locality, as well as at LACMIP locs. 8169, 10882, and 15738, all within the Baker Canyon Member, C. crescentica co-occurs with Phygraea inornata (Packard, 1922).
In his caption, Packard (1922:pl. 26, fig. 4) identified specimen UCMP 12319 as belonging to his Ostrea crescentica. In his write-up of this species (p. 420), however, he did not even mention this specimen. According to the UCMP records, specimen UCMIP 12319 is Packard's paratype of O. crescentica, but it is actually a hypotype of A. parasitica (Gabb, 1864).

Curvostrea baia sp. nov.
Figures 43-57 DIAGNOSIS. Shell size medium, shape narrow spatulate to subfalcate, posteroventral margins rarely with lateral flanges on both valves; left valve lowly convex, thick, very heavy, thins posteriorly and flattens ventrally, and with no attachment scar; right valve thin, lowly convex, covered with numerous fine, flat radial riblets crossed by commarginal lines, anterior margin beveled; left valve with welldeveloped, thick, prismatic/foliated microstructure.
DESCRIPTION. Shell size up to height 82 mm. Shape variable, with two morphs: narrow spatulate and subfalcate. Beaks small, poorly developed, ophistogyrate. Posteroventral margins of both valves rarely with lateral flanges. Adductor muscle scar size medium, elliptical, and located near shell margin just posterior of center of valve. Shell microstructure prismatic/foliated with several, thick, long, and lenticular layers of long (up to 4 mm) perpendicular calcite crystals, separated by few, very thin nonprismatic layers. Left valve: Lowly convex, thin but heavy; valve thinning posteriorly and flattening ventrally. Valve can be slightly wedge shaped, with anterior wall higher than posterior margin. No attachment scar. Right valve: Juvenile subtrigonal, umbo inflated, commissure ventrally wavy, resilifer wide, bourrelets indistinct. Anachomata few, short, and extending ventrally from hinge along both margins for less than quarter of total height of valve. Commissural shelf prominent. Adult lowly convex, thin, and with beveled margins; distinct lip present on anterior side. Valve surface covered by many (approximately 15 per millimeter) fine, flat, radial riblets crossing bands of commarginal growth lines that can be raised slightly, thereby creating microscopic cancellate pattern; ribs variable width and can bifurcate; dorsal and medial parts of valve can have few irregular pustulose or lumpy radial ridges. Anachomata only along posterior margin, short and stout on dorsal half of margin but slit-like and more numerous on ventral half of margin. REMARKS. Nine specimens were examined: one left valve and eight right valves. Preservation is excellent for the prismatic microstructure of the left valves and also for the exterior sculpture on a few of the right valves. Elsewhere, preservation is moderately poor. Several specimens are tightly conjoined, thereby making it difficult to discern the commissure. A few right valves have eroded remnants of the thick left valve. Some of these same specimens consist of a thin right valve and an associated thick internal cast of the left valve. These casts consist of tightly cemented, smooth, and shiny siltstone that resembles shell material. The two morphs are probably ecophenotypic responses to the environment, but they might be related to growth stage. More specimens are needed to confirm which of these differing suppositions is correct.
The hinge area of the left valve interior (Fig. 44) bears a cast of a wide, sideways U-shaped structure that could possibly be the impression of the bourrelets and resilifer of the right valve hinge. The sideways orientation would indicate that there was a sudden shift in the growth direction of the valves.
The new species differs from Ostrea crescentica Packard, 1922, by larger size, narrow-spatulate morph, less falcate individuals, thicker and heavier shell, stronger radial sculpture, and much better development of prismatic layers.
The specimens of C. baia are in silty, very fine grained sandstone. The flattish to lowly convex, dense left valve, as well as the absence of an attachment scar, are indicative that C. baia was a recliner.
ETYMOLOGY. The new species name is derived from baios (Greek, meaning ''little''), for the fine radial sculpture of the new species.
AGE AND DISTRIBUTION. Late early Campanian to Oligocene and widespread during the Cretaceous (Cooper, 2002). The species revised here is the first report of Cretaceous Acutostrea in the PSNA region. Acutostrea is common and widespread in Eocene strata of the PSNA region (Moore, 1987).
Acutostrea taxidonta (Packard, 1922)  EMENDED DESCRIPTION. Shell size up to height 132 mm. Shape highly variable, with five morphs: trigonal, spatulate, suboval, falcate, and irregular. Trigonal and spatulate shapes most common, suboval and falcate shapes uncommon, irregular shapes rare. Trigonal morph mostly small in size but larger ones can be gradational in size and shape with spatulate or falcate morphs. Spatulate morph mostly medium in size but larger ones can be nearly as large as suboval ones (largest morph). Irregular shape small in size, rare, and can fill voids. Ventral half of both valves of all morphs, except irregular, geniculate (bent upward) approximately 408 and also having prominent flexure (sulcus) on medial posterior shell margin. Subequivalved. Resilifer well developed. Adductor muscle scar moderately large, reniform (with pointed end pointing toward hinge), and located near central-posterior margin. Shell microstructure prismatic/foliated. Xenomorphic structure common on left valve and can affect conjoined right valve, as well as interiors of both valves. Left valve: Low to moderately low convex. Beak can project beyond right valve beak. Catachomata commonly only on dorsal half of valve, including adjacent to bourrelets. Catachomata straight, those on posterior part of valve longer. Catachomata on larger valves only near hinge, few in number, and generally weak. Valve interior can have irregularities (bumps) near central and ventral regions. Attachment scar on trigonal specimens small to moderate in size, covering beak, and commonly shows outline of foreign matter (commonly long, straight groove with corresponding right valve exterior arched; interiors of valves also reversely affected). Attachment scar wide and large on spatulate specimens; large on suboval specimens, covering  (Packard, 1922) most of valve but not on beak; commonly small and covering beak and, rarely, half of valve on falcate specimens; attachment scar not obvious on irregular specimens. Right valve: Dorsal half commonly convex but can be flat, posterior half commonly concave to very concave. Exterior surface smooth on nearly all smaller specimens. Commarginal lamellae present on all large specimens but rare on small specimens. Exterior surface of valve can have xenomorphic-caused linear bulge, whereas interior surface can have corresponding hollow area. Valve concave medially. Anachomata short, simple, and present on entire posterior side of valve but becoming weaker and more widely spaced beyond midpoint of valve.
HOLOTYPE REMARKS. One hundred and nine specimens were examined. Preservation is generally very good. Only a few, including the holotype, are known from the uppermost Holz Shale Member. The holotype (Figs. 66-68), a trigonal morph, is a left valve interior whose exterior is embedded in rock matrix. The ventral margin is not complete, but what is present shows incipient bending (Fig. 67). Specimens of this species are common and generally well preserved in the Rosario Formation, in Northern Baja California, in the San Antonio del Mar, Santa Catarina, and Puerto Canoas areas (Fig. 1). The largest specimens, which are from a single locality (LACMIP loc. 15388) in the Puerto Canoas area, have excellent preservation. Only a single specimen was detected from the Moreno Formation in Northern California.
The trigonal morph of A. taxidonta is abundant in siltstone in the Rosario Formation in the Santa Catarina area. The spatulate morph is mainly in very fine to fine-grained sandstones in the Rosario Formation in the San Antonio del Mar area. The suboval morph shape occurs in fine-to medium-grained sandstones in the Rosario Formation in the Punta Canoas area. At LACMIP locs. 22414 and 27263 in the San Antonio del Mar area and at LACMIP loc. 15388 in the Punta Canoas area, both the spatulate and suboval morphs coexist. Acutostrea taxidonta co-occurs with A. parasitica in the Rosario Formation at LACMIP loc. 27263.
The presence of several morphs of A. taxidonta in the Rosario Formation is not incompatible because the basal part of the formation, where specimens of this oyster are abundant, was deposited in a dynamic geologic setting. Miller and Abbott (1989), Lescinsky et al. (1991), and Johnson and Hayes (1993) reported that the basal conglomerate and sandstone units of the Rosario Formation were pervasive and deposited along a long coastline significantly shaped by subduction tectonism. The advancing (transgressive) sea eroded the margins of the coastline and created rocky shore habitats associated with agitated waters, where both the spatulate and suboval morphs of A. taxidonta occur. It is likely that both forms lived in agitated waters, but the suboval forms have a larger attachment surface area because of living in more agitated waters.
Upsection, the basal conglomerate and sandstone units of the Rosario Formation give way to relatively deeper shallow marine units. It is likely that the trigonal morph of A. taxidonta, which is found in siltstones in the Santa Catarina area, preferred these slightly deeper and less agitated waters than where the spatulate and suboval morphs lived. For these calmer water specimens, the attachment scar is smaller because of the lower energy water conditions. Some specimens of the trigonal morph can be noticeably directed posteriorly (Fig. 71), so much so as to grade into the falcate morph (Fig.  77). This gradation occurs at several localities where a sufficient number of specimens (more than 10) were available for comparison (e.g., LACMIP locs. 2857 and 2858 [Rosario Formation, Santa Catarina] and LACMIP loc. 11975 [Tuna Canyon Formation]).
Acutostrea taxidonta is abundant in the Tuna Canyon Formation at LACMIP loc. 11975 in Garapito Canyon, Los Angeles County, Santa Monica Mountains, but preservation is poor because many of these specimens are encased in extremely hard sandstone concretions, and attempts to remove the sandstone matrix result in serious degradation of the shell material. Splitting of the concretions mostly revealed only very poorly preserved interior surfaces of trigonal morphs and a few falcate morphs. On the interior of several of the trigonal left valves is a linear xenomorphic bulge (e.g., Fig. 72). The accompanying right valve (e.g., Fig. 73), which was originally conjoined, has the same linear xenomorphic feature, but it occurs as a groove.
Some conjoined valves of the trigonal morph of A. taxidonta, especially from the Santa Catarina area, also show a grooved, linear xenomorphic attachment scar on the exterior surface of the left valve (e.g., Fig. 58) and a corresponding linear bulge on the exterior of the accompanying right valve (Fig. 59).
The irregular morph of A. taxidonta is represented by rare individuals attached to narrow gastropod shells. An example (Figs. 74, 75) from the Holz Shale Member of the Ladd Formation is a right valve whose exterior surface (Fig. 74), but not its interior surface (Fig. 75), shows the outline of the whorls and ribs of a Turritella gastropod. This oyster grew attached to the inside of an exterior mold of the Turritella specimen, thereby indicating a considerable interval of time between the preservation of the Turritella specimen and the formation of the oyster specimen.
In terms of its considerable morphologic variability, A. taxidonta is very similar to what Cooper (2002:39-43, figs. 6-9) observed for Acutostrea incurva (Nilsson, 1827) from upper Campanian to Maastrichtian strata of Europe, Algeria, Russia, and southeast Africa. Cooper (2002:39) stated that the range in morphology of A. incurva is ''so variable that it almost defies description,'' and he concluded that this variability is dependent on the size of the attachment area. Sundberg (1980:845, Sundberg (1980) is most likely the truncatedspatulate morph of Phygraea inornata (Packard, 1922), known from the Baker Canyon Sandstone Member.  (Komatsu et al., 2002). The species revised here is the first Cretaceous report of Crassostrea in the PSNA region. Crassostrea is represented by several species, some of them of very large size, in Miocene and Pliocene deposits of California and Baja California (Moore, 1987).
REMARKS. This genus is morphologically highly variable, ranging from small size to very large size. Stenzel (1971) reported that Crassostrea is characterized by having no chomata. Sohl and Kauffman (1964:7) reported that Mesozoic Crassostrea species have chomata, whereas Tertiary species do not. Toulmin (1977) reported, however, Eocene Crassostrea with chomata.
Crassostrea, a soft-sediment recliner, is dominant in marginal marine environments but ranges into shallow, nearshore marine situations (Sohl and Owens, 1991:217). Komatsu et al. (2002) reported that this oyster lives today predominantly on muddy bottoms, where a firm basis for attachment is rare. They also reported that individuals of some species are known to have built large colonies to keep their position above the surface of the mud. EMENDED DESCRIPTION. Shell size up to height 190 mm. Shell can be very thick, with left valve up to thickness 44 mm and right valve up to thickness 16.5 mm (not same individual). Shape variable with four morphs: subquadrate, narrow/elongate trigonal, subtrigonal, and rarely crescent. Inequivalved. Elongate posteroventrally. No attachment scar. Resilifer well developed, high and long (up to height 40 mm and length 25 mm), straight or curved (ophistocline). Bourrelets well developed and highly arched, especially ventrally; bourrelets can be equally wide or posterior bourrelet more arched and 2.5 times wider than anterior bourrelet. Adductor muscle scar large, oval with dorsal end ''pointed,'' and posteroventrally located. Shell microstructure prismatic with numerous thick, long lenticular layers consisting of perpendicular large calcite crystals and separated by thinner, long lenticular layers filled with foliated and recrystallized material. Left valve: Convex, ranging from moderately to deeply convex. Valve generally straight-sided; largest specimen with very prominent horizontal, posteroventral bend in ventral region. Posterodorsal area commonly ''hump'' shaped. Overhang of shell can occur on anterodorsal region. Prominent groove extending along anterior side of valve and noticeable depressed areas just ventral of resilifer and bourrelets, with depression ventral of anterior bourrelets deeper. Chomata rare, near hinge. Large left valves can have chevron-shaped radial costae near ventral margin but not affecting commissure, except in minor way. Right valve: Flattish with upward low deflection on ventral area; valve smooth, moderately thick, with wide ligamental tooth.
HOLOTYPE. UCMP 31447, height 113.7 mm, length 72 mm, UCMP loc. 11381, ''Chico Group, Cow Creek'' (Stewart, 1930:131 REMARKS. Twelve specimens were examined. Preservation is generally good, but most are large fragments of left valves. The four morphs of this species most likely represent ecophenotypic responses to changes in the environment, but more specimens are needed to confirm this supposition. All the examined large-size, left-valve specimens from British Columbia and Southern California (Figs. 98, 100, 108) have a moderate to large size (up to diameter 33 mm), conjoined-valved specimen of the mytilid bivalve Lithophaga sp., which bored into the posterodorsal ''hump'' region in or near the umbo. These oyster valves also have small-diameter (up to 4 mm) boreholes, infilled with rock matrix, located elsewhere on them. Gabb (1864:pl. 26, fig. 191) provided only a sketch of the interior of the left valve of the holotype of brewerii, and this sketch shows a projection on the right side of the figure. Stewart (1930:131) reported correctly that this projection is actually another valve, which is in the same hand specimen as the holotype and immediately adjacent to it. Gabb's holotype (Fig. 105) is shown here with the projection missing, and the immediately adjacent valve is shown separately (Fig. 104). The latter is a slender left valve with a narrow, tapered resilifer, which is a very common feature in Crassostrea.
Crassostrea brewerii is the largest and most thick walled Late Cretaceous oyster known from the PSNA region. This species is similar to Crassostrea cusseta Sohl and Kauffman (1964:pl.5, figs. 1-4) of Campanian age from the Gulf Coast of the United States, in terms of the following features: heavy, thick shell consisting of thick prismatic layers alternating with much thinner foliated layers, well-developed resilifer and strongly arched bourrelets with subcardinal cavities, rare chomata, and rough exterior surface. Crassostrea brewerii differs by being smaller (C. cusseta has an average shell height of 377 mm) and in having  (Packard, 1922) wide-spatulate rather than narrow-elongate shape, a shorter bend ventrally in the shell, and a better developed adductor muscle scar.
AGE AND DISTRIBUTION. Early Cenomanian to Maastrichtian and widespread (except in polar seas) (Reeside, 1929;Stenzel, 1971). The species revised here and the new species named below are the first reports of Costagyra in the PSNA region.
Costagyra juveniles were cemented to secondary hard substrate, whereas adults were adapted to a free, reclining mode of life on soft sediment. The thick shells of some species served to increase stabilization on the soft sediment and also helped to protect against predators (Ayoub- Hannaa and Fürsich, 2011). Variations in shell thickness and ornament of Costagyra were possibly related to water energy (Seeling and Bengtson, 1999:758). Its shell microstructure is predominantly prismatic, with some foliated layers (Jaitly et al., 2014).

EMENDED DESCRIPTION.
Shell size up to height 55 mm. Shape rectangular to subfalcate. Inequivalved. Umbonal region tightly spirally coiled posteriorly. Shell microstructure predominantly prismatic with much thinner foliated layers. Left valve: Convex with medial axis coincident with broad, bumpy keel extending posteriorly from umbo to venter; keel can be continuous or with broad sulcus on medial part of valve. Posterior side of valve bearing nine to 10 ribs (can be dichotomous) which extend from umbo to venter and widen toward venter; radial ribs narrow to moderately wide and can be nearly hyotelike near venter; interspaces wide and concave. Attachment area present on small specimens. Right valve: Flat to slightly concave (near hinge) and smoothish, with overlapping spirally arranged growth lamellae. Radial ribs short (approximately eight) along anterior side of first whorl of spiraling umbo, with ribs becoming wider, more widely spaced, and less prominent with increasing size of whorl. Resilifer groove narrows posteriorly and can be nearly filled with shell deposit. Anterior bourrelet thin, coincident with sharp anterior edge of shell, and bearing many short anachomata. Posterior bourrelet forms short projecting ridge adjacent to concavity just ventral of hinge line.  (Packard, 1922).
REMARKS. Sixteen specimens were examined. Preservation is moderately poor, and the radial ribs are very faint on most specimens because of weathering, including the holotype (Fig. 114) DIAGNOSIS. Shell size large, Costagyra with keeled left valve bearing 12 to 13 radial ribs.
DESCRIPTION. Shell size up to height 100 mm. Shape subfalcate to falcate. Very inequivalved, beaks spirally coiled. Shell microstructure predominately prismatic with some much thinner foliated layers. Left valve: Inflated greatly. Radial ribs 12 to 13, wide, can be dichotomous, and with moderately wide interspaces; ribs extend to umbo where partly covered by thin shell with rugose growth lines; ribs become wider ventrally and have development of some hyote-like raised areas; ribs obsolete on some large specimens. REMARKS. Seven specimens were examined. Preservation is moderately good. Two of the specimens are conjoined. Costagyra garza differs from C. californica by having a much larger size, nonrectangular shape, and more radial ribs. The new species is very similar to Costagrya olisipoensis illustrated by Malchus (1990:pl. 10, figs. 6a, 6b) and by Aqrabawi (1993:pl. 4, figs. 3-5, pl. 5, figs. 1, 2). The new species differs by having no hyote spines, a less prominent umbo (i.e., wide and projected), and no ribs on the right valve. Elder and Miller (1993: (Aqrabawi, 1993). The species revised here is the first report of Amphidonte in Northern Baja California, Mexico.
EMENDED DESCRIPTION. Shell size normally up to height 80 mm, rarely up to 100 mm. Shape variable with three morphs: auriform (ear-like), wedge, and short tube, listed in decreasing order of abundance. Auriform morph with shell height approximately 17-70 mm, wedge morph shell height approximately 53-100 mm, and shorttube morph shell height 20-60 mm. Auriform morph gradational in shape with wedge morph and, to lesser degree, with short-tube morph. Valve exteriors smooth, with only growth lamellae (if any). Adductor muscle scar size medium to large; scar shape oval on all morphs, and either central or off-center, near posterior wall. Umbo opisthogyral to various degrees. Hinge area commonly strongly coiled but can be poorly developed and wide. Resilifer not prominent on most specimens. Sigmoidal sinus (notch) can consist of infolded area where chomata bend abruptly in half-circle fashion, affecting both valves near posterior bourrelet, causing margin of the sinus to be wrinkled and forming small depression in left valve and corresponding slight bulge in right valve. Sigmoidal sinus can be ''closed up'' on some specimens. Ligamental area can remain obscure to weak or moderately strong. Chomata small, numerous and cover both interior margins of each valve or can be obscure. Shell microstructure foliated, with lens-shaped layers. Left valve: Attachment surface flat or slightly concave and commonly smoothish on all morphs. Smallest specimens (shell height 17 mm) crescent-shaped with wide umbo, hinge spirally coiled, and resilifer and sigmoidal sinus not obvious or poorly developed. Early adult to mature adult auriform-morph individuals (shell height normally 35-43 mm, but uncommonly can be up to 70 mm) with generally flattish valve, anterior wall slightly to moderately elevated with respect to posterior wall, umbo broad, hinge spirally coiled, resilifer obscure, sigmoidal sinus well developed, and chomata well developed. At approximate shell height 35 mm, auriform-morph anterior wall can become gradationally more elevated (increasing from 21 to 32 mm high, rarely up to 50 mm high), and also nearly vertical to vertical; posterior wall much lower (increasing from 5 to 13 mm high) and much thinner. At approximate shell height 50-58 mm, and continuing to shell height 80 mm, anterior wall elevated enough and thick enough to form wedge morphs. Distinction between specimens intermediate in shape between auriform and wedge morphs is arbitrary because change in the height of anterior wall relative to the posterior wall is gradational. Wedge morph has left valve sloping approximately 308 from highest point toward posterior margin; in addition, umbo spirals posterior and begins to ''close up,'' as does sigmoidal sinus. Anterior wall normally smooth but can be broadly lumpy, with broad sulcus between two lumps. Short-tube morph only known from left valves, anterior wall thin (up to 14 mm high) and more elevated than posterior wall (up to 5 mm high), thereby producing overall partial wedge shape (depending on view). Short-tube morph flat bottomed (well cemented to hardground), resilifer small, sigmoidal sinus area generally obscured, and adductor muscle scar circular. Right valve: Auriform immature valves (less than shell height 20 mm) known only from right valves, flattish to lowly convex, with ligamental area very poorly developed (barely discernible), small indentation to accommodate spiraling of beak, and sigmoidal sinus absent. Auriform adult right valves flat (lid-like), smooth, thin but with some thickening anteriorly, especially on dorsoanterior region. As umbo spirals posterior, ligamental area can narrow and begin to ''close up,'' as does sigmoidal sinus (especially on wedge morphs). Ligamental area rarely very wide and obscure. On some auriform specimens (shell height 70 mm), right valve countersunk into groove of corresponding left valve, with numerous, short, and closely spaced anachomata on right valve margin meshing with chomata on overlapping left valve.
REMARKS. Eighty-five specimens were examined. Preservation is best in the Point Loma Formation near Carlsbad, San Diego County; the Rosario Formation at San Antonio del Mar, Baja California, Mexico; and the Cedar District Formation on western Denman Island, British Columbia, Canada. Elsewhere, preservation is generally good. The short-tube morph (e.g., Figs. 141, 142) is only found in the Point Loma Formation in the Carlsbad area and in the upper Cedar District Formation on western Denman Island. The lectotype of A. parasitica is an early adult-auriform morph, and two views of a plaster cast of the lectotype are shown in Figures 131 and 132. Elder (1991:E11, unfig.) reported the largest specimen of A. parasitica as having a shell height of 100 mm. It is from an unnamed faultbounded stratigraphic rock unit near Loma Prieta [Area 11]. Elder (1991:pl. 4, figs. 9, 17-23) illustrated several morphs of A. parasitica found in this rock unit and referred to them as ''typical'' and ''ovate'' [both same as auriform here], ''ovate with carinate umbilical ridge'' [same as short tube here], and ''elongate.'' Based on the low profile, robust shell, and large attachment surface of many of the oysters, they were adapted to a nearshore, high-energy environment. In the rock unit near Point Loma, they subsequently underwent transport downslope by turbidity currents and into deeper waters, where they accumulated as oyster coquinas (Elder, 1991).
A growth stage series of the auriform morph, ranging in height from 20 to 70.6 mm (Figs. 128-145), is best represented at LACMIP loc. 3966 in the Point Loma Formation, at Madonna Hill in the Carlsbad area, San Diego County [Area 17]. At this locality, the majority of the specimens represent the auriform morph, a few somewhat resemble the wedge morph, and a few are the short-tube morph. At other localities in the immediate vicinity, specimens are represented by the auriform and wedge morphs or by the auriform and those that resemble the wedge morphs. At nearby LACMIP loc. 17423, a large auriform morph (shell height 70 mm) with conjoined valves (Figs. 143-145) was collected.
The presence of growth stages and conjoined valves are strongly indicative that A. parasitica auriform morphs found in sandstones of the Point Loma Formation, Carlsbad area, San Diego County, are in situ or nearly so. Kennedy et al. (2000) interpreted the depositional environment at the Palmer Way localities in the Point Loma Formation to have been an intertidal? to a very shallow, nearshore marine, rocky bottom habitat. The common co-occurrence of auriform and the sturdier looking wedge morphs is suggestive that microenvironments (e.g., possibly protected habitats versus more exposed habitats, respectively) played a role in determining the morph shape. Detailed sedimentologic and paleoenvironmental studies, which might help to understand the variability of morph shapes found at these Carlsbad localities, cannot be made because the specimens were collected from temporary exposures of mediumgrained (on average) sandstone at several localities (e.g., LACMIP 3966, 17423), just a few meters stratigraphically above a basal conglomerate. All these beds were destroyed in subsequent development.
In the Rosario Formation at LACMIP loc. 27263, in the San Antonio del Mar area, Baja California, Mexico [Area 19], both the auriform and wedge morphs are present, but the auriform morph (Fig. 146) is a minor component compared with the wedge morph (Figs. 147, 150). The specimens are robust and heavy, mostly free of sandstone matrix, and dark gray. Many of the specimens have large prominent ''holes''(openings caused by dissolution) coincident with former position of the adductor muscle scar (Figs. 146,147). One hand specimen (Fig. 147) contains three large specimens (height 60-82 mm) of A. parasitica, which solidly encrust one another. The specimen shown in Figure 147 is an auriform morph isolated from the hand specimen, whereas the other two specimens are more wedge shaped. Also at locality 27263, a left valve of a wedge morph of A. parasitica (Fig. 150) is totally cemented to a right valve of a trigonal morph of A. taxidonta. Sedimentologic and paleoenvironmental analyses are so far lacking for rocks at this locality.
Only the wedge morph (e.g.,Figs. 148,149,151,152) is known from LACMIP loc. 22415 in the Williams Formation, Pleasants Sandstone Member, Santa Ana Mountains, Orange County [Area 16]; specimens are not common. The left valves are thick and robust (i.e., sturdy looking). These Bee Canyon exposures are slightly younger than the Pleasants Sandstone Member found elsewhere and contain a shallow-marine fossil assemblage deposited in moderate depths (Saul, 1982:74). Detailed sedimentologic and paleoenvironmental studies are also lacking for this locality. Saul (1986a:29) reported two fragments of a large thick-shelled ostreid from LACMIP loc. 26525 in the lower upper Maastrichtian El Piojo Formation in the vicinity of Lake Nacimiento [Area 13]. The largest fragment (height 68 mm) is a wedge morph. Its smooth exterior has been bored by bivalves, which is a common feature in specimens of A. parasitica found elsewhere. Kirby (1991:80, pl. 3, fig. 1) reported unidentifiable ostreid fragments from LACMIP loc. 14320 in a thin interval of uppermost Maastrichtian strata in the basal San Francisquito Formation in the Warm Springs Mountain area, Los Angeles County [Area 14]. Fossils in this part of the formation are generally not well preserved. He stated that one of the thin-shelled specimens might be a Pycnodonte, but it is Amphidonte parasitica.

REMARKS.
A single and unusual left valve of ?Amphidonte parasitica (Figs. 156, 157) from LACMIP loc. 3966 in the Point Loma Formation, near Carlsbad, San Diego County, Southern California, is similar to the auriform morph of A. parasitica in terms of overall shape and adult size, anterior wall slightly higher than posterior wall, and large subcentral adductor muscle scar. This unusual specimen differs from A. parasitica by having prominent, angularly terminate wide plicae on the anterior wall, as well as obvious bourrelets, and catachomata present on only the dorsal half of both margins of the valve. In addition, the catachomata differ by being shorter, stronger, and similar shapes on both margins of the valve. This unusual specimen might represent another species. Although the presence of plicae on the ?A. parasitica specimen is indicative of placement in Ceratostreon Bayle, 1878, such a placement is unmerited because Ceratostreon has a much more spirally incurved ligamental area, as well as much weaker and longer catachomata.
REMARKS. One hundred and twenty-eight specimens were examined. Both left and right valves are equally represented, and some are conjoined. Preservation of many specimens is excellent. The valves of malleiformis are most similar to those of the type species Pycnodonte (P.) radiata, as described by Stenzel (1971. The similarities are the left valve umbo rising barely above the long, straight dorsal margin; auricles present; radial ribs absent; and concentric puckers and welts absent. The valves of malleiformis differ by not being circular nor subcircular, and its chomata are not vermiculate. Pycnodonte (P.) malleiformis is very similar in morphology to the extant Pycnodonte (P.) taniguchii Hayami and Kase (1992:1076-1082 from shallow depths (20-30 m) in several poorly lighted submarine caves (i.e., cryptic, sheltered habitat) of Miyako and Okinawa Islands, southern Japan. This oyster seems to adhere to cave walls via attachment of the left valve. According to Hayami andKase (1992:1081), P. (P.) taniguchii is closely similar to P. (P.) radiata. Pycnodonte (P.) malleiformis differs from P. (P.) taniguchii by having a much smaller size, much flatter left valve, and absence of a well-defined commissural shelf.  (Stenzel, 1971). The species revised here and the new species named below are the first reports of Cretaceous Phygraea in the PSNA region. Phygraea is represented by several species in Paleocene and lower Eocene strata in California (Squires and Demetrion, 1994).
The posteroventral flange of Phygraea most likely provided stability on soft sediments. Even at single localities there can be much morphologic variation, which represents polymorphic populations rather than allopatric or chronologic species/subspecies (Wilmsen and Voigt, 2006:23).

EMENDED DESCRIPTION.
Shell size up to 66 mm height. Shell thin and smooth. Shape variable with three morphs: narrow, wide, and truncated spatulate. All strongly inequivalved. No auricles. Both valves smoothish. Adductor muscle scar small and off-center. Shell microstructure vesicular, in thin layers. Left valve: Juvenile and early adult specimens of narrow morph with umbo compressed, prominent, and tapered; beak straight, incurved (either anteriorly or posteriorly), and rising well above hinge. Adult narrow-morph specimens rarely have wide umbo, distinct posteroventral flange, and pseudo-auricle on anterodorsal region where attachment area intersects side of valve. Truncated-spatulate morph similar to narrow morph but with truncated umbo; umbo and beak replaced by small to large concavity (bowl-like) or flattened to bulging region representing attachment area on anterodorsal part of shell; ridgelets, welts, or puckers can be present adjacent to attachment area. Attachment scar not obvious. Catachomata on truncate-spatulate morph simple, short, and only on dorsal half of both shell margins.  (Packard, 1922).
REMARKS. Three hundred and fifty specimens were examined. Nearly all of them are left valves, and their preservation is moderately good. Right valves and conjoined valves are scarce and poorly preserved. Specimens are encased in hard siltstone or in poorly sorted silty fine- grained sandstone, and the majority of them are from the Baker Canyon Member. Shell exteriors are decorticated on many specimens. Chomata are rarely preserved, and most are preserved on internal casts. The vesicular microstructure has been filled in with secondary calcite.
Two views of the holotype are figured here (Figs. 174, 175). Packard (1922) mentioned a ''cotype'' of his species but did not assign a catalog number. The whereabouts of this ''cotype'' are unknown. Representative interiors of the valves of P. inornata are shown here for the first time (Figs. 183, 184, 187).
The morphs of P. inornata are not restricted geographically nor stratigraphically, and this information is strongly suggestive that they are the result of ecophenotypic factors rather than evolution (i.e., separate species). For example, these polymorphic populations occur in the Redding Formation, Shasta County, Northern California (LACMIP loc. 24654), and in the Baker Canyon Member, Orange County, Southern California (LACMIP loc. 8195). The relative abundance of each morph, however, can differ from locality to locality. Most of the narrow-morph specimens of P. inornata show no attachment scar, but a rare specimen from locality 8195 (Fig. 172) has a small shell attached to its umbo. Also, most of the wide-morph specimens do not show an attachment scar. A particularly wide specimen (Figs. 176-178) from LACMIP loc. 10887 in the Baker Canyon Member has a moderately large attachment scar creating a pseudo-auricle. The truncated-spatulate morph specimens all show attachment scars on the left valve. The shape of P. inornata, therefore, was largely controlled by whether there was attachment to a foreign object.
As mentioned earlier, P. inornata co-occurs with C. crescentica and C. californica at various localities in the Baker Canyon Member of the Ladd Formation, Santa Ana Mountains, Orange County, Southern California.
Phygraea arida sp. nov. Figures 11, 188-199 DIAGNOSIS. Shell size large. Shape variable but predominantly circular. Shell smooth, flattish, moderately thick, and heavy. Left valve with slight genticulation posterior to umbo region and with tongue-like flange extending from posterocentral region. Right valve low convex anteriorly, concave posteriorly, with well-developed commissural shelf.
DESCRIPTION. Shell size up to height 95 mm. Shell moderately thick and heavy. Shape variable with three morphs: circular, truncated spatulate, and subtrigonal. Circular morph common, other morphs rare. Umbo and beak mostly improminent, except projected beak on subtrigonal morph. Dorsal margin long, rarely short. Valves smooth (no radial ribs). Adductor muscle scar moderate size, circular, nearly central but somewhat near hinge. Catachomata and anachomata only near hinge and vermiculate (lath-like). Commissural shelf prominent, delimited proximally by circumferential curb. Shell microstructure vesicular (''foamy'') with several thin to thick layers (individually up to 8 mm thick) separated from each other by thin horizontal lamellae. Left valve: Juvenile shell area of circular morph inflated; adult circular morph with prominent tongue-like flange extending from posterocentral region, and anteroventral part of interior of valve concave; commissural shelf on larger specimens close to muscle scar and to prominently raised posterior of muscle scar. Right valve: Lowly convex (inflated) along anterior side and concave posteroventrally; umbo area can be swollen. Adductor muscle scar near posterior, anachomata along dorsal anterior and dorsal posterior regions. Commissural shelf occurs as raised narrow curb along anterior margin and occurs as much broader swollen area along most of posterior side, including posterior side of adductor muscle scar.
HOLOTYPE REMARKS. Thirty specimens were examined, including one conjoined-valved specimen. Preservation is generally excellent. Some of the Moreno Formation specimens are fragments. All the specimens (10) from the upper Cedar District Formation are small size, whereas those from California are much larger. Phygraea arida co-occurs with A. parasitica in the Cedar District Formation at the ''White House'' site on western Denman Island, British Columbia, Canada.
Most specimens of P. arida were probably soft-sediment recliners, based on absence of any attachment scar. A small-sized, single left valve from the upper Cedar District Formation, however, is strongly cemented to an echinoid spine (Fig. 199).
The new species differs from P. inornata by having a much larger and heavier shell, well developed commissural shelf on many specimens, discernible ''foamy'' texture, orbicular shape on most specimens, stronger vermiculate chomata, no overhanging or twisted beaks, and no radial sulcus.
The new species is very similar to the Pycnodonte (Pycnodonte) gigantica (Solander, 1766:36, fig. 88). Solander's species was originally named Ostrea gigantica and was the type species of Gigantostrea Sacco (1897:14, pl. 4) from Eocene strata in Italy. The new species differs from P. (P.) gigantica by having a tongue-like flange extending centroposteriorly from the left valve. Stenzel (1971) equated Gigantostrea with Pycnodonte (Pycnodonte), but his usage is not followed here.
ETYMOLOGY. The new species name is derived from aridus (Latin, dry), in reference to Laguna Seca (Spanish, dry lake) Creek.

PALEOGEOGRAPHY AND PALEOCLIMATE COMMENTS
The PSNA region makes up a large part of Kauffman's (1973: fig. 1) Cretaceous bivalve-based ''Northeast Pacific Subprovince'' of the ''North Pacific Province,'' which was part of the ''North Temperate Realm.'' A temperate realm for this area has been corroborated by faunal studies of a wide array of mollusk groups (Saul, 1986b;Kiel, 2002), including neritid and cypraeoidean gastropods , nerineid gastropods (Saul and Squires, 1998;Saul and Squires, 2002), volute gastropods (Saul and Squires, 2008), and opine bivalves (Squires and Saul, 2009). These studies provide ample evidence that the ''Northeast Pacific Subprovince,'' in the area of this present report, experienced warm-temperate waters during much of the Late Cretaceous. In addition, Coralliochama orcutti, a warm-water rudist bivalve (Saul, 1986b), cooccurs with A. parasitica in four formations in PSNA: 1) Gualala Formation, Anchor Bay Member, Mendocino County (Elder et al., 1998); 2) Williams Formation, Pleasants Sandstone Member (LACMIP loc. 22415, Bee Canyon, Santa Ana Mountains, Orange County); 3) basal Point Loma Formation (vicinity of LACMIP loc. 17421, Carlsbad, San Diego County); and 4) Rosario Formation (LACMIP loc. 27263, San Antonio del Mar, Baja California, Mexico). The warm-temperate A. parasitica is one of most common PSNA oysters of Campanian to Maastrichtian age and has the widest geographic range, from British Columbia to Northern Baja California. In Europe, Amphidonte is also a warm-temperate oyster and not reported from Tethyan (tropical) locales in that region (Aqrabawi, 1993).
None of the PSNA Late Cretaceous oyster genera are endemic to the ''Northeast Pacific Subprovince.'' They are all widespread genera, and, using Kauffman's (1973:359) bivalve categories, Rastellum [¼ Arctostrea], Crassostrea, and Pycnodonte are ''truly cosmopolitan forms.'' Although Stenzel (1971 reported the geographic distribution of Pycnodonte (Pycnodonte) to be worldwide, Hayami andKase (1992:1076) noted that Cretaceous, as well as Paleogene, species of this oyster are known almost exclusively from low-middle latitudinal regions; namely, western Europe, Crimea, north and west Africa, south India, and Gulf Coast regions. The PSNA region can now be added to this list. Phygraea was widely distributed in both temperate and tropical (Tethyan) waters, and this oyster has diminished value for paleogeographic purposes (Aqrabawi, 1993).

BIODIVERSITY COMMENTS
During Late Cretaceous time, the world experienced greenhouse climate conditions, and the late Cenomanian and early Turonian interval coincided with the Cretaceous thermal maximum and highest sea levels. The Turonian-Coniacian turnover event, which coincided with a global drop in sea level, was a cool time that persisted during Coniacian time. Although there was a modest warming in early Santonian time, cool-water conditions returned during the late Santonian. During Campanian time, there were fluctuations in sea surface temperatures and sea level, but there was an overall warming trend and a sea level rise that culminated during early Maastrichtian time (Forster et al., 2007:fig. 2;Miller et al., 2011:fig. 4;Linnert et al., 2014: fig. 2).
The PSNA Late Cretaceous oyster fauna (Fig. 2) had a high biodiversity level (five species) during Turonian time. This was followed by a dramatic drop (zero species) during the Coniacian, a low level (one species) near the end of the Santonian, and a mostly gradual buildup during the Campanian. The highest biodiversity level (six species) occurred in the late early to early late Maastrichtian, followed by a continuing high level (five species) during the late Maastrichtian. This overall biodiversity pattern is very similar to that of Late Cretaceous neritid and cypraeoidean gastropods in the PSNA region .
A rigorous analysis of the causes of the observed biodiversity pattern in the PSNA Late Cretaceous oysters is beyond the scope of this paper. Some possible explanations include 1) global changes in ocean surface water temperatures, 2) global changes in sea level, 3) closure of seaways, 4) tectonic influences associated with the active plate margin that existed in the PSNA region, and 5) collecting/sampling artifacts. The available evidence is indicative, however, that warm surface waters during the Turonian, Campanian, and Maastrichtian would have favored the thermophilic oysters, and the accompanying high sea levels would have provided more habitats and the best potential for burial of the nearshore oysters. The abrupt drop of diversity at the end of the Turonian was most likely related to the influx of cooler waters. The cooler surface waters, reduced potential for habitats, and pervasive on-going erosion of nearshore deposits associated with the lower sea levels during the Coniacian and Santonian could have been responsible for the paucity of oyster taxa. The PSNA oyster species that originated during the Campanian went extinct at various times, thereby indicating more gradual and short-lived changes in temperature.
In the PSNA region, there was a reoccurrence of the warm-water Turonian Curvostrea-Costagyra-Phygraea association in the late early to early late Maastrichtian. This reoccurrence is very strongly indicative that warm temperature and high sea level were largely responsible for the highest diversities of PSNA oysters.
In his work on Late Cretaceous oysters in Jordan, Aqrabawi (1993) reported a biodiversity pattern similar to that of the PSNA region, in that there are no oyster taxa of Coniacian age and there is low biodiversity in the Santonian, in contrast to much higher biodiversity before and after these times. He also reported a similar pattern for Late Cretaceous oysters from northwest Europe. For both areas, he provided no evidence as to what caused the observed patterns, although he did mention, for northwest Europe, that transgressions and regressions were responsible.
The four new species are Oscillopha popenoei, Curvostrea baia, Costagyra garza, and Phygraea arida. The one unnamed species is Rastellum sp., aff. R. macropterum (J.D.C. Sowerby, 1825). Oscillopha popenoei is the first known occurrence of this genus in North America, and Rastellum is the first known occurrence of this genus in the PSNA region.
Genera represented by a single species are Oscillopha, Rastellum, Acutostrea, Crassostrea, and Pycnodonte (Pycnodonte). Those represented by two species are Curvostrea, Costagyra, possibly Amphidonte, and Phygraea. All the oyster genera were globally widespread, and none were endemic to the PSNA region.
Amphidonte has the longest geologic range in the PSNA region, from early Campanian to questionably latest Maastrichtian. The species of Oscillopha, Rastellum, Curvostrea, Acutostrea, Crassostrea, Costagyra, and Phygraea were relatively short lived. Phygraea inornata is the most abundant Late Cretaceous PSNA oyster. Crassostrea brewerii is the largest species, with its thick and heavy large specimens, which were commonly bored by large specimens of the bivalve Lithophaga. The PSNA Turonian species of Curvostrea, Costagyra, and Phygraea are much smaller than their early Maastrichtian species.
Nearly every Late Cretaceous PSNA oyster species has several morphs, which are most likely ecophenotypic responses related to changes in the paleoenvironments or related to the presence or absence of an attachment scar. In the case of A. taxidonta, the outline of its shell is dependent on the size of the attachment scar. Trigonal specimens have a small attachment scar, spatulate specimens have a large attachment scar, and oval specimens have an even larger attachment scar. It is likely that the larger the size of the attachment scar, the more agitated the waters were that the specimens lived in. In the case of P. inornata, whether or not there was attachment to a foreign object directly affected the shape of the individuals. Other species (e.g., Crassostrea brewerii) might also be represented by ecophenotypes, but the number of specimens is too low to confirm this. Xenomorphism was most commonly observed in valves of Acutostrea taxidonta. A growth series was detected for A. parasitica.
The highest biodiversity levels of PSNA oysters (five to six species), which occurred during warm-temperate times and high sea levels (Turonian and late early to early late Maastrichtian), stand in stark contrast to a much lower biodiversity level (either no species or only one) during cooler times (Coniacian and Santonian, respectively). In the PSNA region, there was a reoccurrence of the Turonian warm-water Curvostrea-Costagyra-Phygraea association during the warm-water times of the late early to early late Maastrichtian. The PSNA oyster biodiversity pattern needs further study, but the available evidence is strongly indicative that global fluctuations in surface water temperature and contemporaneous sea level changes were most likely the controlling forces.