194 The True Sharks 
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 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 Lamnidea.—To the family of Lamnide 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, asin themackerels. This 
form is especially favorable for swift swim- 
ming, and it has been independently de- 
veloped in the mackerel-sharks, as in the | 
macker<ls, 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 Fa die nasi ue Rais 
Kadiak Island it destroys schools of na cuspidata Agassiz. 
salmon, and along the coasts of Japan, and re ( ae 
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. 
Fic. 186.—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. 
