The nucleus appears to be either dextral or sinistral indifferently. The sinistral or dextral shell is imperforate, conic-oblong and solid. Within a normally-dextral species, rare individuals may develop sinistral coiling. Most species of sea snail are dextral, though some are naturally sinistral. Vergence, similarly to fold pairs, is sometimes classified as sinistral or dextral. A NW-SE trending sinistral shear belt separates northern and southern crustal terranes. The final stages of the Caledonian orogeny involved sinistral strike-slip faulting. The most common was the lightning whelk, which has a left-handed or sinistral spiral. This spreading is also tied to sinistral strike-slip faulting at 39 Ma. The shell of this species is sinistral in coiling and almost planispiral in shape. Separate records for sinistral (left-handed) shells and obviously rostrate cowries (family Cypraeidae) are also included. All species within family Planorbidae have sinistral shells. The shell is sinistral, subglobose, subdiscoidal, hyaline and very thin. These species though sinistral are different species. The aperture is sinistral and relatively small.