In distinction from the true fishes, the gills are differently formed, adnate by their outer margin, there are no membrane bones about the head, the ova arc very large, the ventral fins are provided with claspers, there is no trace of air bladder, the arterial bulb has three series of valves, there is a spiral valve in the rectum, the upper jaw is formed of palatal elements, the typical jawbones of the fish being undevel oped. The lower jaw is also different in struc ture from that of the true fishes.
The existing .elasmobranchs are known as sharks, rays and chimaeras. The vast majority of the known species are extinct. There are two strongly marked sub-classes among the elas mobranchs, the Selachii or sharks and rays, and the Holocephali or chimeras. In the Selachii there are five to seven gill-openings, the jaws are distinct from the skull, and the teeth are distinct. In the Holocephali there is but one external gill-opening, the jaws are coalescent with the skull, and the teeth are united to form are broad and fold-like, the notochord is appar ently not segmented, the tail is short and keeled, well specialized, its tip abruptly turned upward. There are no spines, the teeth are small, with many cusps. There is probably but one family, the Cladoselachidez (extinct), Cladoselache fyleri, a large elongate shark from the Devonian of Ohio, is the best known species.
The Acanthodii are small sharks with a spine at the front of each fin except the caudal. The teeth are minute or wanting, and the skin is covered with small checker-like plates. There are three families, Acanthoessidos (extinct), with one dorsal fin, with two, and the Ischnacanthidee (extinct), small sharks found from the Devonian to the Permian.
The Ichthyotomi have the pectoral fin de veloped as an archipterygium or jointed limb with a fringe of rays on one or both sides. The dorsal fin extends along the back, and on the head is a first dorsal preceded by a long spine. There are two well-marked families, Pleuracanthidee and Cladodontida, abundant in the Carboniferous and Permian, but now ex tinct.
The Notidani or Diplospondyli have the notochord imperfectly segmented by vertical partitions, and the gill-clefts are six or seven in number instead of five, as in other sharks. Most of the species are extinct, the teeth being found in the rocks from the Jurassic to the present time. Two families are represented, the Hexanchidee and the Chlamydoselachidee, the latter ell-shaped sharks of the open sea, chiefly about Japan.
In the large order of Asterospondyli the ver tebra are strengthened by secondary plates of calcified tissue, which radiate outward from the small primitive cylinder. In these typical
sharks there are five gill-slits, two dorsal fins and one anal fin.
In the most primitive group, the sub-order Cestraciontes, the dorsal fins are each armed with a spine, the numerous teeth are small and mostly blunt, differing in form in different parts of the jaw, and the vertebra are imper fectly formed. A curious fact in geological dis tribution is that a multitude of early types of shark disappear in the Permian or toward the bony plates or lamella. Both groups are very old in geologic times, having been separated at least since the Devonian. For this and other reasons some writers prefer to regard the sharks and chimeras as separate and co-ordinate groups or classes.
We may without serious violence divide the sharks and rays into six orders; namely, Pleu ropterygii, Acanthodii, Ichthyotona, Notidani, Asterospondyli, and Tectospondyli, the first three of these being confined to Paleozoic We may regard the Pleuropterygii or the allies of Cladoselache as the most primitive, and therefore as standing first in an ascending series.
In this group the pectoral and ventral fins end of Paleozoic time. Only cestraciont sharks are known to have any representatives in the Triassic, and this group may be ancestral to all modern sharks.
Of the Cestraciontes the Paleozoic families of Cochtiodontide (extinct) and Orodontidte (extinct), known mainly by the teeth, occur in the Lower Carboniferous. In some and prob ably all of these forms the dorsal fins were each armed with a spine. The Edestidee (ex tinct), known only from coiled whorls of fused teeth, are doubtless closely related to these forms. These are found in the coal measures. The principal living family of Cestraciontes, the Heterodontidee, begins in the Permian, and is represented by five living species all in the Pacific Ocean, the longest known being the Port Jackson shark of Australia (Heterodontus philippi). We may Lcre mention two families of sharks of uncertain relationship, the species confined to the Carboniferous Age. These are the Pitalodontida (extinct), with blunt teeth and some of them with broad fins like rays, and the Psammodontida (extinct), known from the blunt teeth only. Still more uncertain is the group of Tamiobatiche (extinct) from the Devonian of Kentucky, resembling a ray, but probably a primitive offshoot from the sharks.