452 FISHES CHAP. 



which is as long as the rest of the body (Fig. 258). Its teeth 

 are of moderate size, triangular in shape, and without serrations. 

 The " Thresher " has a wide distribution, being abundant in the 

 Atlantic and Pacific Oceans, besides being the commonest of the 

 larger Sharks frequenting the British coasts. It grows to a 

 length of 15 feet, of which the tail forms at least one-half. 

 Quite inoffensive to man, the Thresher feeds on the shoals of 

 smaller Teleosts, such as Pilchards, Herrings, and Sprats. When 

 feeding it swims in gradually diminishing circles round the 

 shoal, splashing the water with its long tail, and keeping its 

 victims so crowded together that they become an easy prey. A 

 remarkable Lamnoid Shark (Mitsukurina owstoni}? which has 

 the snout produced into a "long, flat, flexible, leaf -like blade," 



FIG. 258. The Thresher Shark (Alopecias vulpes). (From Jordan and Evermann.) 



somewhat resembling that of Polyodon, but narrower and more 

 pointed, and has protractile jaws and large spiracles, is found 

 in deep water near Yokohama, and may prove to be generically 

 identical with the Cretaceous Shark Scapanorhynchus.' 2 



Lamnoid Sharks are not certainly known to have existed 

 until the Upper Cretaceous formations, in which, as well as in 

 different Tertiary deposits, teeth indistinguishable from those of 

 the existing genera Lamna, Odontaspis, and Carcharodon are 

 found. The interesting genus Carckarodon has one extinct 

 species in the Cretaceous and several others distributed in 

 Tertiary formations in nearly every part of the world. The 

 teeth of some of the Tertiary species measure 5 inches along 

 the margin and 4 inches across the base, and it is evident that 

 they belonged to Sharks so gigantic as completely to dwarf the 

 existing species. That these giant Lamnidae have only recently 



1 D. S. Jordan, California Acad. Sci. (3), Zool. i. 1898 ; Bashford Dean, Science 

 (N.S.), xvii. 1903, p. 630. 



- Smith Woodward, Ann. Mag. Nat. Hist. (7), iii. 1899, p. 487. 



