On the other hand, if we regard the paired fins as parts of a lateral fold of skin, we find primitive sharks to bear out our conclusions. In Cladoselache of the Upper Devonian, the pectoral and the ventral fins are long and low, and arranged just as they might be if Balfour's theory were true. Acanthoessus, with a spine in each paired fin and no other rays, might be a specialization of this type or fin, and Climatius, with rows of spines in place of pectorals and ventrals, might be held to bear out the same idea. In all these the tail is less primitive than in the Ichthyotomi. On the other hand, the vent in Cladoselache is thought by Dean to have been near the end of the tail. If this is the case, it should indicate a very primitive character. On the whole, though there is much to be said in favor of the primitive nature of the Ichthyotomi (Pleuracanthus) with the tapering tail and jointed pectoral fin of a dipnoan, and other traits of a shark, yet, on the whole, Cladoselache is probably nearer the origin of the shark-like forms.
The relatively primitive sharks called Notidani have the weakly ossified vertebræ joined together in pairs and there are six or seven gill-openings. This group has persisted to our day, the frilled shark (Chlamydoselachus) and the genera Hexanchus and Heptranchias still showing its archaic characters.
Here the sharks diverge into two groups, the one with the vertebræ better developed and its calcareous matter arranged star-fashion. This forms Hasse's group of Asterospondyli, the typical sharks. The earliest forms (Orodontidæ, Heterodontidæ) approach the Notidani, and so far as geological records go, precede all the other modern sharks. One such ancient type, Heterodontus, including the bullhead shark, and the Port Jackson shark, still persists. The others diverge to form the three chief groups of the cat-sharks (Scyliorhinus, etc.), the mackerel-sharks (Lamna, etc.), and the true sharks (Carcharhias, etc.).
Fig. 256.—Mackerel-shark or Salmon-shark, Lamna cornubica (Gmelin). Santa Barbara, Cal.
In the second group the vertebræ have their calcareous matter arranged in rings, one or more about the notochordal center. In all these the anal fin is absent, and in the process of specialization the shark gradually gives place to the flattened body and broad fins of the ray. This group is called Tectospondyli. Those sharks of this group with one ring of calcareous matter in each vertebra constitute the most primitive extreme of a group representing continuous evolution.
From Cladoselache and Chlamydoselachus through the sharks to the rays we have an almost continuous series which reaches its highest development in the devil rays or mantas of the tropical seas, Manta and Mobula being the most specialized genera and among the very largest of the fishes. However different the rays and skates may appear in form and habit, they are structurally similar to the sharks and have sprung from the main shark stem.
Fig. 257.—Star-spined Ray, Raja stellulata Jordan & Gilbert. Monterey, Cal.
The Chimæras.—The most ancient offshoot from the shark stem, perhaps dating from Silurian times and possibly separated at a period earlier than the date of any known shark, is the group of Holocephali or Chimæras, shark-like in essentials, but differing widely in details. Of these there are but few living forms and the fossil types are known only from dental plates and fin-spines. The living forms are found in the deeper seas the world over, one of the simplest in structure being the newly discovered Rhinochimæra of Japan. The fusion of the teeth into overlapping plates, the covering of the gills by a dermal flap, the complete union of the palato-quadrate apparatus or upper jaw with the skull and the development of a peculiar clasping spine on the forehead of the male are characteristic of the Chimæras. The group is one of the most ancient, but it ends with itself, none of the modern fishes being derived from Chimæras.