1 86 



PLEURA CANTHODII 



The chief points of interest in the paired fin are the breadth of the 

 base, which shows no sign of constriction 

 from the body -wall either in front or 

 behind ; and the correlated slight con- 

 centration of the radials. Yet the radials 

 are numerous, and are stouter at the pre- 

 axial border than at the postaxial, where 

 considerable concentration may have taken 

 place without producing the posterior 

 notch characteristic of the fins of most 

 other fish. Only vague indications of 

 girdles can be made out ; the pelvic 

 radials seem to have iremained separate 

 in the body -wall without fusing to a 

 basipterygium (Fig. 155); the pectoral 

 radials fuse to some extent, at all events, 

 to form basal pieces. 



No claspers have been found in the 

 Cladoselachii ; we must therefore suppose 

 that this sub-class diverged from the main 

 stem before the Chondrichthyes had ac- 

 quired them. The antiquity of the strata 

 in which the Cladoselachii occur, their 

 general proportions and simplicity of 

 structure, their dentition, amphistylic 

 jaws, and paired fins, all favour the view 

 ft *ey are primitive. The strongly 

 heterocercal tail, and the circumorbital 



Fin. 154. 



Cladoselaclic Fylfri, .Newb. 

 Caudal tin. (After Dean, from 

 A. S. Woodward.) D, tin-mem- 



Fio. 155. 



Cladoselache Fyleri, Newb. Left pectoral and pelvic fins. (After Dean, from A. S. Wood- 

 ward.) B, basal cartilages within the body-wall ; D, dermal tin-membrane with dermal rays ; 

 R, endoskeletal radials. 



