are so stiffened with mineral deposits 

 that microscopic examination and a 

 knowledge of the developmental 

 processes involved in cartilage and 

 bone formation are necessary todeter- 

 mine that cartilage, not bone, makes 

 up the skeletons. For this reason, we 

 often speak of the "backbone" of a shark 

 when referring to the hard, cartilagi- 

 nous (but bonelike) vertebrae that com- 

 prise the structure that places the 

 members of the class Chondrichthyes 

 in the Vertebrata. 



Fossil records of primitive fishes 

 ancestral to present-day sharks, 

 skates, rays, and chimaeras, date back, 

 perhaps a quarter of a billion years, to 

 the Devonian period (mid-Paleozoic 

 era). Moreover, representatives of all 

 of the presently existing families of 

 cartilaginous fishes have been found in 

 the fossil deposits of the early Ter- 

 tiary period (early Cenozoic era)-- 

 good evidence that these fish families 

 have been on earth at least a thousand 

 times longer than the human family. 

 This long geological history also indi- 

 cates that the cartilaginous fishes can 

 maintain themselves successfully in 

 spite of changing environment and vary- 

 ing conditions of competition, and that 

 they have been remarkably stable (in 

 the biological sense of resistance to 

 evolutionary change) since at least the 

 Tertiary period. 



Zoological definitions adequate to 

 set off the classes of vertebrates from 

 one another with precision, or to define 

 the subdivisions of cartilaginous fishes, 

 are necessarily long. To find the de- 

 tailed and exact definitions required for 

 taxonomic work, the reader is referred 

 to one or more of the appropriate cita- 

 tions at the end of this circular. For 

 general purposes, the following re- 

 marks should be sufficient to separate 

 the major groups: 



Sharks usually have the shape of a 

 "typical fish"--a fusiform shape--and 

 most of them are densely covered -with 

 small, toothlike scales called denticles 

 which give the skin its rough sand- 

 paperlike feeling. In addition, all sharks 

 have from five to seven gill slits or 

 external gill openings on each side of 

 the head or neck. 



Skates and rays have a more-or- 

 less flattened appearance, and all ex- 

 cept a few are either wholly or partially 

 covered with denticles or spines, which 

 may be scattered irregularly or ar- 

 ranged in definite patterns. All skates 

 and rays have five gill slits on each 

 side of the lower surface of the head- 

 neck region. 



Chimaeras, like sharks, are fusi- 

 form in shape. Unlike sharks, they 

 are usually smooth skinned, although 

 a few denticles appear on the young 

 and may persist in some adults, and 

 they have only one external gill open- 

 ing on each side. 



Although some sharks and rays 

 enter fresh water for short periods, 

 and a few custonnarily enter fresh water 

 to give birth to their young, most 

 species are exclusively marine. One 

 species of shark is said to be es- 

 tablished as a permanent resident of 

 Lake Nicaragua, but this is exceptional. 

 Most chimaeras inhabit deep water, 

 and all live in strictly oceanic environ- 

 ments. 



Reproductive processes are ex- 

 tremely variable in the class Chon- 

 drichthyes. Many members of this 

 class are ovoviviparous and bring forth 

 living young that have been hatched 

 from eggs inside the oviducts of the 

 mother. Once hatched, the embryos 

 are nourished in a variety of ways. 

 Some members are viviparous, or 

 nearly so, and bring forth living young 

 that have been nourished by substances 

 in the mother's blood stream trans- 

 ferred to the developing embryo through 

 a placenta. Some are oviparous and lay 

 eggs encased in horny or leathery cap- 

 sules, which are deposted on the ocean 

 floor. Regardless of the developmental 

 process, the number of eggs produced 

 by a single cartilaginous fish is very 

 small compared with the number of eggs 

 produced by a single bony fish. But the 

 cartilaginous fishes are able to main- 

 tain their numerical status quo, because 

 their young, at birth or hatching, are 

 much larger than the newly hatched 

 young of most kinds of bony fishes and 

 are, therefore, better able to defend 

 themselves against predators. A 



