88 THE FOOD OF ANIMALS 



SHARKS AND RAYS (Elasmobranchs) 



Typical Sharks. — The order of Elasmobranch fishes, including 

 Sharks, Rays, and their allies, is essentially a predaceous one. 

 In such a typical member of the group as the Blue Shark {Car- 

 charias glaucus) (see vol. i, p. 285) the jaws are found to be armed 

 with numerous rows of sharp triangular teeth, forming very effi- 

 cient offensive weapons. The large mouth is situated far back 

 on the under side of the head, a fact which accounts for the well- 

 known habit this and other sharks have of turning over on their 

 backs before seizing prey. This particular species commonly 

 attains the length of 15 feet, while allied species may be as much 

 as 25 feet long. Much larger proportions are exhibited by the 

 Rondeletian Shark (Carcharodon Rondeleiii), which has a wide 

 distribution throughout the hotter parts of the ocean, and for 

 which a length of 40 feet is recorded. It must not, however, 

 be supposed that the size of the prey is proportionate to the size 

 of the shark, or that all large species are dangerous to man. The 

 size and character of the teeth afford a much safer guide to the 

 habits. For example, the Basking Shark (Cetorhinus i7taxi-mus), 

 which is the biggest of its kind to be found in the North Atlantic, 

 attaining as it does a length of 30 feet or more, is perfectly harm- 

 less if left alone, and feeds upon comparatively small animals. In 

 accordance with this its numerous conical teeth are of small size. 



"Shepherding" of Prey. — Another interesting species with rela- 

 tively small teeth is the Thresher {Alopecias vulpes), distinguished 

 by the extreme length of the upper lobe of its tail, which is longer 

 than the rest of the body. The food consists of fish which swim 

 in shoals, such as herrings, and the common name indicates the 

 function of the long tail, which is used as a flail to beat the water, 

 so that the frightened prey may be induced to herd closely together, 

 with the result that their capture is easy. 



Saws. — Teeth are by no means the only weapons of offence 

 possessed by the Shark tribe. In Saw- Fishes [Prisiidce), for 

 example, which are widely distributed through the hotter parts 

 of the sea, and are known to ascend some of the Indian rivers, 

 the snout is drawn out into a long flattened process armed on 

 either side with a row of powerful tooth-like scales, that have 

 nothing to do with the ordinary jaw -teeth (fig. 369). Such 



