The Scyliorhinidae include the dog-fishes, most of which are small bottom-living fishes, spotted, marbled or banded, with rounded fins and with small cuspidate teeth ; they are distin guished from the Lamnidae and Carchariidae in that they are not viviparous, but lay large heavily-yolked eggs enclosed in oblong horny cases, with threads at each corner that serve for attach ment. The Orectolobidae are distinguished by a pair of grooves running through the upper lip and connecting the nasal sacs with the mouth. Orectolobus of Australia and Japan includes species with a broad, flat head, margined with skinny flaps, much as in the angler (Lophius) ; most of the other species are bottom-living and have the appearance of dog-fishes, but one is pelagic and is the largest of all sharks, for it is said to reach a length of 7o feet. This shark (Rhinodon) known as the whale-shark, or at the Seychelles as the "chagrin," has the same numerous small teeth and long gill-rakers as the basking shark, which it resembles in habits.
A small group of sharks is distinguished by having six or seven gill-openings on each side, and a single dorsal fin, opposed to the anal. The six-gilled shark (Hexanchus griseus) is said to attain a length of over 25 ft.; there are a few records from British seas. The frilled shark (Chlamydoselachus anguineus) is an eel-shaped shark known from deep water off Japan and in the North Atlantic.
The third group of sharks has five gill-openings (six in Plio trema), and two dorsal fins, each of which is typically preceded by a spine; most of these are bottom-living forms. The genus Heterodontus includes about seven species from the Indian and Pacific oceans, distinguished by having small teeth in the front of the jaws and large blunt teeth, suitable for crushing shell-fish, at the sides; the egg-case is conical, with spiral ridges. H. Philippi is the Port Jackson shark. The piked dog-fishes (Squalus) inhabit the north and south temperate zones ; they have no anal fin ; the points of the small teeth are deflected laterally, so that the inner margin forms a cutting edge. Squalus acanthias is very abundant
in British waters, and off the Atlantic coast of North America; it reaches a length of 4 ft.; it is a great nuisance to fishermen, eating fish caught in lines or in nets. All the Squalidae are vivi parous; two noteworthy species of this family, both without fin spines, are the spiny shark (Echinorhinus), a large bulky shark with spiny tubercles on the skin, and the Greenland shark (Somni osus) of the Arctic seas, which reaches a length of 25 ft., has conical teeth in the upper jaw and cutting teeth in the lower, and is active and voracious, often attacking whales. Many Squalidae (Centrina, Centrophorus, etc.) are inhabitants of deep water, and off Portugal are caught by long lines at depths of 400 or 500 fathoms. Closely related to the Squalidae are the saw sharks, Pristiophorus of the Indo-Pacific and Pliotrema of South Africa, with the snout produced into a flat blade bearing a series of teeth on each side. Also related to the Squalidae are the angel fishes or monks (Squatina) with flattened body, large wing-like pectoral fins, and terminal mouth with conical teeth; about ten species of this genus are known, mainly from temperate seas.
Few sharks are valued as food, but the dried fins are used by the Chinese for the preparation of gelatine, and for culinary pur poses. Shagreen, used for polishing wood, is the skin of those kinds that are covered with numerous close-set pointed denticles. In recent years extensive fisheries for sharks have developed, starting in Carolina, and spreading to Africa, Australia, etc. The skins are made into leather of high quality; the oil obtained from the liver is said to be excellent, and the rest of the shark is used in the preparation of products such as glue, meal, etc. (See