226 PALAEONTOLOGY 



member of this group (Helicoprion, Permo-Carboniferous) 

 these old teeth are rolled up into a spiral of four or more 

 whorls. 



Some sharks and the rays, instead of swallowing their 

 prey whole, grind it in their mouths : in these, the teeth 

 have lost their sharp points and compressed shape, and 

 form a mosaic with a rough surface, extending over the 

 palate and part of the floor of the mouth as well as the 

 jaws (Fig. 63, f). 



No attempt can be made here to describe the details 

 of the endoskeleton of Chondrichthyes, but a few words 



n.o. 



FIG. 64. TOOTH STRUCTURE. 

 (Diagrammatic vertical section.) 



d, Dentine; e, enamel; c, crown; p.c., pulp-cavity; n.o., nutritive 

 opening ; r, root. 



may be given to the general form of the body. This is 

 elongated, tapering towards both ends, laterally com- 

 pressed in the nectic (swimming) forms, depressed in the 

 benthic (bottom-living) skates. Besides the two pairs 

 of fins (answering to the limbs of land-vertebrates) there 

 are a variable number of median fins, spaced out along 

 the whole of the dorsal edge, but only on that part of the 

 ventral edge that lies behind the paired fins. The end 

 of the tail is bent up and combines with a median 

 ventral fin to form the " heterocercal caudal fin," charac- 

 teristic not only of all cartilaginous fishes and some 



