Page:Encyclopædia Britannica, Ninth Edition, v. 21.djvu/802

Rh 776 SHARK fishes are distinguished from the other British littoral sharks by each of the two dorsal fins being armed in front FIG. fl. Teeth of Acan- thias vulgaris. FIG. 8. Egg-shell of same fish (x i). I., external view; IT., section ; a and 6, the two spiral ridges ; e, cavity for the ovum. by an acute spine. They do not possess an anal fin. Their teeth are rather small, placed in a single series, with the point so much turned aside that the inner margin of the tooth forms the cutting edge (fig. 9). The spiny dog- fish are of a greyish colour, with some whitish spots in young specimens, and attain to a length of 2 or 3 feet. They are viviparous, the young being pro- duced throughout the summer months. It is stated that in the northern islands of Great Britain they are dried for food, and that their livers yield a large quantity of oil. Finally, we have to notice among the littoral sharks the "Angel-Fish" or "Monk-Fish" (Rhina squatina), which, by its broad flat head and ex- panded pectoral fins approaches in general appearance the rays. It occurs in the temperate seas of the southern as well as the northern hemisphere, and is not uncommon on sandy parts of the coast of England and Ireland. It does not seem to exceed a length of 5 feet, is not used as food, and is too rare to do any perceptible injury to other fish. t is said to produce about twenty young at a birth. Pelagic Sharks. All these are of large size, and some are surpassed in bulk and length only by the larger kinds of cetaceans. Those armed with powerful cutting teeth are the most formidable tyrants of the ocean and dangerous to man, whilst others, which are provided with numerous but very small teeth, feed on small fishes only or marine invertebrates, and are otherwise almost harmless and of a timid disposition, which causes them to retire into the solitudes of the open sea. On this account we know very little of their life ; indeed, some are known from a few individuals only which have accidentally come ashore. All pelagic sharks have a wide geographical range, and many are found in all seas within the limits of the equatorial zone, some being almost cosmopolitan. All seem to be viviparous. Of the more remarkable forms which we propose to notice here the genus most abundantly represented in species and individuals is Carcharias. Perhaps nine-tenths of the sharks of which we read in books of travel belong to this genus. Between thirty and forty species have been distinguished, all of which are found in tropical seas. They are the sharks which so readily attach themselves to sailing vessels, following them for weeks, and thus exhibit- ing an endurance of muscular power scarcely found in any other class of animals. Others affect more the neighbour- hood of land, congregating at localities where nature or the vicinity of man provides them with an abundant supply of food. One of the most common species, and one of those which extend far into the temperate zones, is the Blue Shark (Carcharias glaucus), of which small specimens (4 to 6 feet long) are frequently caught on the south coasts of England and Ireland. Other species of Carcharias attain a length of 25 feet. The mouth of all is armed with a series of large flat triangular teeth, which have a sharp, smooth, or serrated edge (fig. 10). FIG. 10. Dentition of the Blue Shark (Carcharias glaucus). The single teeth are of the natural size. Galeocerdo is likewise a large shark very dangerous to man, differing from the preceding chiefly by having the outer side of its teeth deeply notched. It has long been known to occur in the North Atlantic, close to the Arctic Ocean (G, arcticus), but its existence in other parts has been ascertained within a recent period ; in fact, it seems to be one of the most common and dangerous sharks of the Indo-Pacific, the British Museum having obtained specimens from Mauritius, Kurrachee, Madras, and the west coast of Australia. Hammerheaded Sharks (Zyn&na) are sharks in which the anterior portion of the head is produced into a lobe on each side, the extremity of which is occupied by the eye. The relation of this unique configuration of the head to the economy of the fish is unknown. Otherwise these sharks resemble Carcharias, and are equally formidable, but seem to be more stationary in their habits. They occur in all tropical and subtropical seas, even in the Mediterranean, where Z. malleus is by no means rare. In the Indian Ocean it is common, and Cantor states that specimens of this species may be often seen ascending from the clear blue depths of the ocean like a great cloud. The Porbeagles (Lamna) differ from the preceding sharks in their dentition (fig. 11), the teeth being large, lanceo- late in shape, not adapted for cutting, but rather for seizing and holding the prey, which consists chiefly in fish. These sharks are therefore not dangerous FIG. 11. Teeth of Lamna.