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 Lamnidez.—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 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 
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 ite ioe S reneh of hoa 
Kadiak Island it destroys schools of na cuspidata Agassiz. 
; : Oligocene. Family 
salmon, and along the coasts of Japan, and [amnide. (After Nich- 
especially of Europe and across to New lon.) 
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. 
Fre. 136.—Mackerel-shark, Iswropsis dekayi Gill. Pensacola, Fla. 
In the closely related genus, Jsurus, the mackerel-sharks, 
this cusp is wanting, while in Jsuropsis 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. 
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