Fig. 337.—Hammer-head Shark, Sphyrna zygæna L. Hindustan. (After Day.)
Suborder Cyclospondyli.—In this group the vertebræ have the calcareous lamellæ arranged in a single ring about the central axis. The anal fin, as in all the tectospondylous sharks and rays, is wanting. In all the asterospondylous sharks, as in the Ichthyotomi, Acanthodei, and Chimæras, this fin is present. It is present in almost all of the bony fishes. All the species have spiracles, and in all are two dorsal fins. None have the nictitating membrane, and in all the eggs are hatched internally. Within the group there is considerable variety of form and structure. As above stated, we have a perfect gradation among Tectospondyli from true sharks, with the gill-openings lateral, to rays, which have the gill-opening on the ventral side, the great expansion of the pectoral fins, a character of relatively recent acquisition, having crowded the gill-openings from their usual position.
Family Squalidæ.—The largest and most primitive family of Cyclospondyli is that of the Squalidæ, collectively known as dogfishes or skittle-dogs. In the Squalidæ each dorsal fin has a stout spine in front, the caudal is bent upward and not keeled, and the teeth are small and varied in form, usually not all alike in the same jaw.
Fig. 338.—Dogfish, Squalus acanthias L. Gloucester, Mass.
The genus Squalus includes the dogfishes, small, greedy sharks abundant in almost all cool seas and in some tropical waters. They are known by the stout spines in the dorsal fins and by their sharp, squarish cutting teeth. They are largely sought by fishermen for the oil in their livers, which is used to adulterate better oils. Sometimes 20,000 have been taken in one haul of the net. They are very destructive to herrings and other food-fishes. Usually the fishermen cut out the liver, throwing the shark overboard to die or to be cast on the beach. In northern Europe and New England Squalus acanthias is abundant. Squalus sucklii replaces it in the waters about Puget Sound, and Squalus mitsukurii in Japan and Hawaii. Still others are found in Chile and Australia. The species of Squalus live near shore and have the gray color usual among sharks. Allied forms perhaps hardly different from Squalus are found in the Cretaceous rocks and have been described as Centrophoroides. Other genera related to Squalus live in greater depths, from 100 to 600 fathoms, and these are violet-black. Some of the deep-water forms are the smallest of all sharks, scarcely exceeding a foot in length. Etmopterus spinax lives in the Mediterranean, and teeth of a similar species occur in the Italian Pliocene rocks. Etmopterus lucifer,[150] a deep-water species of Japan, has a brilliant luminous glandular area along the sides of the belly. Other small species of deeper waters belong to the genera Centrophorus, Centroscymnus, and Deania. In some of these species the scales are highly specialized, pedunculate, or having the form of serrated leaves. Some species are Arctic, the others are most abundant about Misaki in Japan and the Madeira Islands, two regions especially rich in semi-bathybial types. Allied to the Squalidæ is the small family of Oxynotidæ with short bodies and strong dorsal spine. Oxynotus centrina is found in the Mediterranean, and its teeth occur in the Miocene.
Fig. 339.—Etmopterus lucifer Jordan & Snyder. Misaki, Japan.
Family Dalatiidæ.—The Dalatiidæ, or scymnoid sharks, differ from the Squalidæ almost solely in the absence of dorsal spines. The smaller species belonging to Dalatias (Scymnorhinus, or Scymnus), Dalatias licha, etc., are very much like the dogfishes.