RECENT SHARKS 89 
“primitive” eeatures, 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, Sgualus acanthias,L. 3. X%. (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- 
Pig. 95. — The thrasher shark, Alopias vulpes (Gmel.), Bonap. 2. x Ys. Atlantic. 
creased : predatory forms, such as Squalus, Alopias, Lamna 
(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- 
