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 (JMtsukurina owstont),' which has 
the snout produced into a “long, flat, flexible, leaf-lke blade,” 
Fic. 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.” 
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 Carcharodon 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 
1D. 8. Jordan, California Acad. Sci. (3), Zool. i. 1898 ; Bashford Dean, Science 
(N.S.), xvii. 1903, p. 630. 
2 Smith Woodward, Ann. Mag. Nat. Hist. (7), iii. 1899, p. 487. 
