There are over 980 species of cartilaginous fish. A stingray is a type of cartilaginous fish. This allows more efficient locomotion among these negatively buoyant cartilaginous fish. In turn, the bony fish came from the Acanthodii class of bony and cartilaginous fish. There are 48 species and six subpopulations of cartilaginous fish assessed as endangered. Placoid scales are found on cartilaginous fish including sharks. Cartilaginous fish don't have swim bladders. Saw sharks and sawfish are cartilaginous fish possessing large saws. As in cartilaginous fish, the upper part of the duct forms the epididymis. Stingrays are a group of sea rays, which are cartilaginous fish related to sharks. As a cartilaginous fish, shark tissues usually do not fossilize. Cartilaginous fish are considered to have evolved from acanthodians. Chondrostei are primarily cartilaginous fish showing some degree of ossification. For cartilaginous fish this occurs through a specialized organ called the Ampullae of Lorenzini. Cartilaginous fish, such as sharks and rays, have also simple, and presumably primitive, skull structures.