IV. VERTEBRATA: PISCES, SELACHIL 571 



Sub Order II. SQUALI (Euselachii). Normal sharks, with cylindrical 

 bodies, free thoracic fins, heterocercal tail, lateral gill slits. Most of them 

 are fast swimmers and are rapacious, the teeth being usually pointed, with 

 sharp or toothed edges, but in some the teeth are pavement-like and are 

 used for crushing shell fish. The numerous families are distinguished by 

 vertebral characters, number of dorsal fins, presence of nictitating mem- 

 brane, etc. In the GALEID.E, in which the nictitating membrane is 

 present, belong, besides the dog-sharks (Mustelus * and Galeus*), the largest 

 of all sharks, Carcharinus* some of which have man-eating reputations. 

 The hammer heads (Zygcena *) are closely allied. The mackerel sharks 

 Lamna*) and the great white * man-eater,' Carcharodon,* lack nictitating 

 membranes. All of the foregoing have star-shaped figures in the verte- 

 brae (p. 570). In the dog-fishes, represented by Acanthias vulgaris* (or 

 Squalus acanthias, fig. 598), there is a spine in front of each dorsal fin. 



Sub Order III. RAI^E. In the skates the body is flattened horizontally 

 fig. 599), and the pectoral fins, also flattened, are united to the sides of 



FIG. 599. Bawi batw, male, ventral view. (After Mo'bius and Heincke ) B, ventral, 

 Br, pectoral fin; B, rostrum; a, anus; c copulatory part of ventral: to sill clefts; 

 wi, mouth; /i, nostril; between them the oronasal groove. 



the body, the union usually extending clear to the tip of the snout, and 

 frequently back to the pelvis, giving the body a rhombic appearance from 

 above. The animals swim by undulating motions of these fins. They 

 mostly lie quiet on the bottom, and hence the lower surface is white, 

 the upper colored. The union of the fins to the side has resulted in trans- 



