Fig. 328.—Scapanorhynchus lewisi Davis. Family Mitsukurinidæ. Under side of snout. (After Woodward.)
Family Alopiidæ, or Thresher Sharks.—The related family of Alopiidæ contains probably but one recent species, the great fox-shark, or thresher, found in all warm seas. In this species, Alopias vulpes, the tail is as long as the rest of the body and bent upward from the base. The snout is very short, and the teeth are small and close-set. The species reaches a length of about twenty-five feet. It is not especially ferocious, and the current stories of its attacks on whales probably arise from a mistake of the observers, who have taken the great killer, Orca, for a shark. The killer is a mammal, allied to the porpoise. It attacks the whale with great ferocity, clinging to its flesh by its strong teeth. The whale rolls over and over, throwing the killer into the air, and sailors report it as a thresher. As a matter of fact the thresher very rarely if ever attacks any animal except small fish. It is said to use its tail in rounding up and destroying schools of herring and sardines. Fossil teeth of thresher-sharks of some species are found from the Miocene.
Family Pseudotriakidæ.—The Pseudotriakidæ consist of two species. One of these is Pseudotriakis microdon, a large shark with a long low tail, long and low dorsal fin, and small teeth. It has been only twice taken, off Portugal and off Long Island. The other, the mute shark, Pseudotriakis acrales, a large shark with the body as soft as a rag, is in the museum of Stanford University, having been taken by Mr. Owston off Misaki.
Family Lamnidæ.—To the family of Lamnidæ proper belong the swiftest, strongest, and most voracious of all sharks. The chief distinction lies in the lunate tail, which has a keel on either side at base, as in the mackerels. This form is especially favorable for swift swimming, and it has been independently developed in the mackerel-sharks, as in the mackerels, in the interest of speed in movement.
Fig. 329.—Tooth of Lamna cuspidata Agassiz. Oligocene. Family Lamnidæ. (After Nicholson.)
The porbeagle, Lamna cornubica, known as salmon-shark in Alaska, has long been noted for its murderous voracity. About Kadiak Island it destroys schools of salmon, and along the coasts of Japan, and especially of Europe and across to New England, it makes its evil presence felt among the fishermen. Numerous fossil species of Lamna occur, known by the long knife-like flexuous teeth, each having one or two small cusps at its base.
Fig. 330.—Mackerel-shark, Isuropsis dekayi Gill. Pensacola, Fla.
In the closely related genus, Isurus, the mackerel-sharks, this cusp is wanting, while in Isuropsis the dorsal fin is set farther back. In each of these genera the species reach a length of 20 to 25 feet. Each is strong, swift, and voracious. Isurus oxyrhynchus occurs in the Mediterranean, Isuropsis dekayi, in the Gulf of Mexico, and Isuropsis glauca, from Hawaii and Japan westward to the Red Sea.