SHARK, the name generally given to the larger kinds of Selachians (q.v.) of the order Pleurotremi, the smaller kinds being known as dog-fishes. Typical sharks are active and piscivorous, swimming near the surface in warm seas, and are generally bluish or greyish in colour; the body is of the normal fish shape, the snout pointed, the crescentic mouth placed on the under side of the head, and there is a series of separate gill-openings on each side; the teeth are often sharp-edged and triangular; the fins are pointed and the end of the tail is strongly upturned. Owing to the position of the mouth a shark may have to turn over to seize a man swimming at the surface, but this is not its normal method of feeding. Some large pelagic sharks have minute teeth and feed on plankton, and among those that live at the bottom there are many divergences from the type described above ; they are generally less active, and have rounded fins and the end of the tail less upturned; some are stout and blunt-headed, others flattened, others eel-like; many are spotted, banded or marbled. The mouth of these bottom-living forms is often transverse, and their teeth are often small and cuspidate, but cutting, piercing and crushing teeth occur. The spiracles, vestigial gill-openings placed behind the eyes, are small in pelagic sharks, but larger in the bottom-living forms, in some (Squatina, Orectolobus) as large as in the rays.
To the family Lamnidae belong several large pelagic sharks, widely distributed in warm seas. One of the largest, swiftest, and most voracious is the ereat white shark or man-eater ( Carcharo don), which reaches a length of 4o ft.; this is somewhat similar in form to a tunny, the body being stout and rounded, the snout pointed, and the tail slender and keeled on each side ; the mouth is crescentic, and the teeth are large, triangular, with sharp ser rated edges; on each side five separate gill-openings appear in front of the large, falcate pectoral fins ; the first dorsal fin is moderate, the second small and opposite the anal; the powerful caudal fin is lunate, the lower lobe being nearly as large as the upper, which is supported by the upturned end of the tail. This
shark feeds on fishes and other marine animals; a young sea lion, weighing ioo lb., was found in the stomach of one 3o ft. long. Teeth found fossil in tertiary strata, and others dredged in the Pacific ocean indicate the former existence of a Carcltarodon about 90 ft. long. The porbeagles (Lamna) are similar in form to the great white shark, but have narrower piercing teeth. The thresher (Alopias) is more slender, and is remarkable for the great length of the upper lobe of the tail-fin; it swims near the surface and feeds on fishes, and has been observed to strike them with blows from the tail. The family Lamnidae also includes the basking shark (Cetorhinus maximus), which takes its name from its habit of basking at the surface. It is distinguished by the large mouth, minute conical teeth and very wide gill-clefts, the inner openings of which are guarded by series of long, slender gill rakers. It reaches a length of 4o ft., but is quite harmless, feeding on minute organisms. Formerly it was hunted by harpooning off Ireland, for the sake of the oil from the liver.
The numerous species of the Carchariidae are mainly tropical, pelagic and piscivorous sharks, with sharp triangular teeth, dis tinguished from the Lamnidae by having the last one or two gill openings above the pectoral fin. The tiger shark (Galeocerdo tigrinus) is one of the largest and most dangerous. The hammer heads (Sphyrna) are peculiar in having the front part of the head produced outwards on each side, with the eyes at the ends of these extensions. The topes (Galeus) and hounds (Mustelus) are small bottom-living Carchariids with small teeth, which are serrated in the topes, blunt and forming a pavement in the hounds; both genera are represented in British seas.