KECEXr SHARKS 



89 



'* primitive " features, like its eel-like shape, may partly be 

 due to its environment : its resemblance, moreover, to the 

 Pleuracanth has since been found to be of a superficial 

 character. Notidanus, on the other hand, adds to its 

 primitive characters the presence of no less than seven 



Fig. 94. — The horned dog-fish, Squalus acanthias, L. cf . X 5. (After GOODE 

 in U. S. F. C.) Atlantic. 



gill slits, — a feature which morphologists generally are 

 inclined to regard as of great significance. 



The many forms of recent sharks have certainly not 

 diverged widely from the stem of descent which Notidanus 

 may well represent : they retain the sub-cylindrical body 

 form, specializing more or less to environment ; in deep- 

 sea genera the body length appears proportionally in- 



Fig. 95. — The thrasher shark, Alopias viilpes (Gmel.), Bonap. ?. X is. Atlantic. 



creased : predatory forms, such as Squalns, Alopias, Lmnna 

 (Figs. 94, 95, 96), acquire great size and strength, travel 

 great distances, and are enabled to become cosmopolitan. 

 Among the minor details to which their evolution has 

 been carried, may be noted : the pattern, size, and arrange- 



