The genus Scyllium includes the true Dog-Fishes (Fig. 254). The species are coast Fishes of small or moderate size, and are widely distributed in temperate and tropical seas, at depths not as a rule exceeding 400 fathoms. Two species, S. canicula and S. catulus, are common on the British coasts, living near the bottom and feeding on Crustaceans and Molluscs. An allied form, Pristiurus, is also common in European and British waters. Chiloscyllium is a widely-distributed genus ranging from the Cape of Good Hope through the Indian Ocean to the coasts of Australia, China, and Japan. Stegostoma tigrinum of the Indian Ocean attains a length of 10 to 15 feet, and is remarkable for its handsome coloration of dark bands on a yellow ground, which has suggested the name of Tiger- or Zebra-Shark. The pelagic genus Ginglymostoma has the terminal portion of the tail bent upwards, and grows to a length of 6 to 12 feet. It is represented by species in the Indian Ocean and the tropical parts of the Atlantic (West Indies and the west coast of Mexico). Crossorhinus includes species of large size, some of which are 10 feet long. They are ground-sharks, frequenting the coasts of Australia and Japan, which lie on the bottom watching for their prey, and in accordance with this habit their coloration closely resembles that of their surroundings.[[529]] A large North Atlantic Shark (Pseudotriakis microdon), of which only two specimens are known, one taken on the Portuguese coast, and the other, 10 feet in length, off Long Island, on the Atlantic coast of North America, has the general characters of the Scylliidae, except that the first dorsal fin is opposite the interval between the pectoral and pelvic fins. Some Scylliidae live at great depths, Scyllium (Scylliorhinus) profundorum having been obtained from a depth of 816 fathoms in the North Atlantic.[[530]]
Fig. 254.—A female Dog-Fish (Scyllium canescens), from the south-western coast of South America. (From Günther.)
Most of the fossil Scylliidae belong to existing genera. The earliest known representatives of the family occur in the Upper Jurassic (Lithographic Stone of Bavaria), where the extinct genus Palaeoscyllium, a near ally of the existing Scyllium, and Pristiurus, are found, nearly complete. Scyllium itself ranges from the Cretaceous through the different Tertiary formations. A species of Chiloscyllium has been recorded from the Miocene Tertiaries, and detached teeth of Ginglymostoma from the Eocene of Belgium and North America. An extinct genus (Mesiteia), which is found in the Upper Chalk of Mount Lebanon and the Upper Eocene of Monte Bolca, is remarkable for the enclosure of its lateral sensory canals in a series of incomplete calcified rings, as in the Holocephali.
Fam. 8. Carchariidae.—Sharks with two dorsal fins, the first in front of the pelvic fins and the second opposite the anal fin, both devoid of spines. Tail without lateral keels. Preoral rostrum elongated. Mouth crescentic. Eyes with nictitating membranes. Spiracles small or absent. Vertebrae asterospondylic. Teeth usually consisting of a single triangular cusp, with smooth, trenchant, or serrated margins, rarely with basal cusps; generally with an axial cavity when fully developed. Viviparous. The family comprises about twenty genera, and approximately sixty species; found in all seas, often in mid-ocean. Amongst the more important genera may be mentioned Carcharias (Carcharhinus), Galeocerdo, Triakis, Thalassorhinus, Galeus, Mustelus and Scylliogaleus.
Fig. 255.—The Blue Shark (Carcharias glaucus). (From Müller and Henle.)
Species of Carcharias are found in nearly all tropical and subtropical seas. The genus is a somewhat comprehensive one, and groups of its species have been distinguished as sub-genera under the names of Prionodon, Hypoprion, Scoliodon, Aprionodon,[[531]] etc. One of the most widely distributed of the thirty to forty species is the Blue Shark, C. (Prionodon) glaucus (Fig. 255), of the Atlantic and Pacific Oceans, which may grow to a length of 25 feet, although the young forms not infrequently captured in British waters do not exceed 6 to 8 feet. It is a slender, swift, pelagic Shark, of a slaty-blue colour above and white underneath, and a voracious hunter of other Fishes. C. nicaraguensis, a Shark about 7 feet long, is confined to Lake Nicaragua and its outlet the Rio San Juan, and is one of the very rare strictly freshwater Sharks. Galeocerdo is a large Shark found in temperate and tropical waters, but one species, G. arcticus, is confined to Arctic seas. The variegated G. tigrinus, or West Indian Tiger-Shark, is said to reach a length of 15 to 20 feet. The genus Galeus includes the small Sharks commonly known as "Topes," which are common in nearly all tropical and temperate seas. The British species, G. canis, which ranges from 4 to 6 feet in length, is a bottom-feeding Fish, preying on Molluscs, Crustacea, Star-Fish, and small Fishes. The various species of Mustelus, or "Hounds," resemble the Topes in their habits and distribution. Living principally on Molluscs and Crustaceans, the dentition has lost the trenchant, unicuspidate type characteristic of most other Carchariidae, and is adapted for crushing and grinding, the teeth being flat, without cusps, and arranged in pavement-like rows. Two species, M. vulgaris and M. laevis, are abundant on the coasts of Europe and the British Isles. Scylliogaleus, which combines the general characters of Mustelus with nostrils similar to those of a Scyllium, is known only from a single specimen from the coast of Natal.[[532]]
The Carchariidae are comparatively modern Sharks. No undoubted remains are known earlier than the Eocene, in which, as in the succeeding Miocene and Pliocene deposits, they are represented principally by their characteristic teeth. The extinct fossil genera are few in number, and so far as their dentition is concerned they differ but little from their living allies.
Fam. 9. Sphyrnidae (Hammer-head Sharks).—In their general characters the Hammer-head Sharks agree with the Carchariidae. They are distinguished, however, by the remarkable shape of the head, which is prolonged into two conspicuous lateral lobes, supported internally by corresponding cartilaginous outgrowths from the post-orbital and the lateral ethmoidal or nasal regions of the skull, with the eyes at their distal extremities, and the nostrils in relation with their anterior margins. One genus and five species.