Fig. 326.—Lower jaw of Cochliodus contortus Agassiz. Carboniferous. (After Zittel.)
Concerning the teeth of various fossil sharks, Dr. Dean observes: "Their general character appears to have been primitive, but in structural details they were certainly specialized. Thus their dentition had become adapted to a shellfish diet, and they had evolved defensive spines at the fin margins, sometimes at the sides of the head. In some cases the teeth remain as primitive shagreen cusps on the rim of the mouth, but become heavy and bluntish behind; in other forms the fusion of tooth clusters may present the widest range in their adaptations for crushing; and the curves and twistings of the tritoral surfaces may have resulted in the most specialized forms of dentition which are known to occur, not merely in sharks but among all vertebrates."
In this neighborhood belongs, perhaps, the family of Tamiobatidæ, known from the skull of a single specimen, called Tamiobatis vetustus, from the Devonian in eastern Kentucky. The head has the depressed form of a ray, but it is probably a shark and one of the very earliest known.
Suborder Galei.—The great body of recent sharks belong to the suborder Galei, or Euselachii, characterized by the asterospondylous vertebræ, each having a star-shaped nucleus, and by the fact that the palato-quadrate apparatus or upper jaw is not articulated with the skull. The sharks of this suborder are the most highly specialized of the group, the strongest and largest and, in general, the most active and voracious. They are of three types and naturally group themselves about the three central families Scyliorhinidæ, Lamnidæ, and Carchariidæ (Galeorhinidæ).
The Asterospondyli are less ancient than the preceding groups, but the modern families were well differentiated in Mesozoic times.
Among the Galei the dentition is less complex than with the ancient forms, although the individual teeth are more highly specialized. The teeth are usually adapted for biting, often with knife-like or serrated edges; only the outer teeth are in function; as they are gradually lost, the inner teeth are moved outward, gradually taking the place of these.
We may place first, as most primitive, the forms without nictitating membrane.
Family Scyliorhinidæ.—The most primitive of the modern families is doubtless that of the Scyliorhinidæ, or cat-sharks. This group includes sharks with the dorsal fins both behind the ventrals, the tail not keeled and not bent upward, the spiracles present, and the teeth small and close-set. The species are small and mostly spotted, found in the warm seas. All of them lay their eggs in large cases, oblong, and with long filaments or strings at the corners. The cat-sharks, or roussettes, Scyliorhinus canicula and Catulus stellaris, abound in the Mediterranean. Their skin is used as shagreen or sandpaper in polishing furniture. The species of swell-sharks (Cephaloscylium) (C. uter, in California; C. ventriosus, in Chile; C. laticeps, in Australia; C. umbratile, in Japan) are short, wide-bodied sharks, which have the habit of filling the capacious stomach with air, then floating belly upward like a globefish. Other species are found in the depths of the sea. Scyliorhinus, Catulus, and numerous other genera are found fossil. The earliest is Palæoscyllium, in the Jurassic, not very different from Scyliorhinus, but the fins are described as more nearly like those of Ginglymostoma.
Close to the Scyliorhinidæ is the Asiatic family, Hemiscylliidæ, which differs in being ovoviviparous, the young, according to Mr. Edgar R. Waite, hatched within the body. The general appearance is that of the Scyliorhinidæ, the body being elongate. Chiloscyllium is a well-known genus with several species in the East Indies. Chiloscyllium modestum is the dogfish of the Australian fishermen. The Orectolobidæ are thick-set sharks, with large heads provided with fleshy fringes. Orectolobus barbatus (Crossorhinus of authors) abounds from Japan to Australia.
Another family, Ginglymostomidæ, differs mainly in the form of the tail, which is long and bent abruptly upward at its base. These large sharks, known as nurse-sharks, are found in the warm seas. Ginglymostoma cirrhatum is the common species with Orectolobus. Stegostoma tigrinum, of the Indian seas and north to Japan, one of several genera called tiger-sharks, is remarkable for its handsome spotted coloration. The extinct genus Pseudogaleus (voltai) is said to connect the Scyliorhinoid with the Carcharioid sharks.