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, Pseiidotriakis acrales, 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 Lamnidse. — To the family of Lavni-idcc 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 coniubica, known 

 as salmon-shark in Alaska, has long been 

 noted for its murderous voracity. About ^^.^^^ ^^g.-Tooth or Lam- 

 Kadiak Island it destroys schools of na cuspidata Agassiz. 

 11 ii .u £ T J Oligocene. Fainilv 



salmon, and along the coasts of Japan, and Lamnidce. (After Xirh- 



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 dekaiii Gill 



Pensarola, 



In the closely related genus, Isitriis, 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 2o to 25 feet. Each is strong, swift, and voracious. 



