The True Sharks 



537 



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 aerates, 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 Lamnidae. To the family of Lamnidce 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 swim- 

 ming, and it has been independently de- 

 veloped in the mackerel-sharks, as in the 

 mackerels, in the interest of speed in move- 

 ment. 



The porbeagle, Lamna cornubica, known 

 as salmon-shark in Alaska, has long been 

 noted for its murderous voracity. About ^ 



J FIG. 329. Tooth of Lam- 



Kadiak Island it destroys schools of na cuspidata Agassiz. 



salmon, and along the coasts of Japan, and (After 



especially of Europe and across to New olson -) 

 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. 



