36 GIANT FISHES 



BLUE SHARKS. 

 (Genus Carcharinus.) PI. II b ; Fig. 19. 



The teeth either stand erect or are set obliquely in the 

 jaws; each tooth has a single, strong, sharp point, which is 

 either smooth or finely saw-edged (Fig. 59). There are no 

 spiracles. The second dorsal fin and the anal fin are both 

 very small. There is a pit at the root of the caudal fin, which 

 has a distinct lower lobe. The coloration is always uniform, 

 and is usually grey or bluish-grey on the back, shading away 

 to white on the lower parts ; in certain species some or all of 

 the fins are tipped with black. 



The largest species (Great Blue Shark) grows to a length of 

 25 feet or more ; many others attain to a length of 10 to 15 

 feet. 



This is a large genus of Sharks, containing many species, 

 and some authorities group these into four or more distinct 

 genera or subgenera, distinguished from one another mainly 

 by the size and character of the teeth. Blue Sharks are to 

 be found in almost all tropical and subtropical waters. The 

 Great Blue Shark (C. glaucus) is cosmopolitan in its distri- 

 bution, and small individuals are not uncommon on the 

 British coasts ; the Black-finned or Black Shark (C. melanop- 

 terus) occurs in the Indian and Pacific Oceans ; the Dusky 

 Shark (C obscurus) is found in the North and Middle Atlantic : 

 and the Cub Shark or Requiem (C. lamia) abounds in the 

 Mediterranean and in the tropical parts of the Atlantic. These 

 are four of the better-known species. The Zambesi Shark 

 (C. zambesensis) , which has been captured 120 miles from the 

 iiver mouth, and the Ganges Shark (C. gangeticus), common in 

 the Ganges, Tigris and other large rivers, are examples of 

 species that habitually entei rivers. One species (C. nicara- 

 guensis) is unique among Sharks in being confined entirely to 

 fresh water, and is found only in Lake Nicaragua and its 

 outlet the Rio San Juan. 



These Sharks, as their lithe, streamlined bodies and uniform 

 coloration suggest, are mostly dwellers in the open sea, and 

 pass a roving, predatory life in the upper layers of the water. 

 Whether disporting themselves at the surface in the sunlight, 



