152 



SELACHII 



developed in front of the dorsal fins in Ccntrina, CentroscyUium, Acanthias, 

 and Caitrophorus. The radials of these fins are much concentrated, and 

 fused proximally to a large basal resting on the vertebral column, as in 

 Cestraciontidae (Fig. 50). In the other genera the fin - skeleton is 

 similarly modified, but the spines appear to have been lost. Occasionally 

 (Acanthias, Fig. 50) may be seen in front of the dermal spine traces 

 of radials, or perhaps of neural spines. The Spinacids are mostly 

 viviparous (p. 134). Laemargus, on the contrary, lays its eggs before 

 they are fertilised (Burckhardt [72], Helbing [206-7]). 



Acanthias, Risso (Squalus, L.), and Centrophorus, M. and H., widely 

 distributed in temperate seas, date back to the Cretaceous epoch. 



Fig. 114. 



A, Acanlhiii.i vuhjaris, Kisso ; and B, ventral view of the head of the same (after Day, 

 modified). C, Zygaena tudes, Val. a.f, anal, c.f, caudal, d.j] dorsal, p./, pectoral, and pv, pelvic 

 fin ; cl, clasper ; a.s, dorsal spine ; c, eye ; m, mouth ; n, nostril ; sp, spiracle. 



Oentrina, Cuv. ; Mediterranean and Atlantic ; Miocene, Europe. Sjpwiewc, 

 Cu v., Scymnus, Cuv., Echinorhinus, Blainv. ; Atlantic and Mediterranean ; 

 Pliocene, Europe. CentroscyUium, M. and H., Laemargus, M. and H. ; N. 

 Atlantic. 



Family Pristiophoridae. Pristiophorus has an elongated, flat 

 rostrum armed with a series of large denticles, or teeth, at its edge, 

 reaching to the angles of the mouth (Fig. 115). Below the rostrum 

 hang two pairs of sensory tentacles. The large spiracles are crescentic. 

 Pliotrema is remarkable in the possession of six pairs of branchial slits 

 (Regan [347]). The teeth are small and conical. The pectoral fins are 

 large, but well marked off at their base from the body. The vertebrae 

 are tectospondylous, and the tail has lateral keels. 



The remarkable resemblance between this family and the Pristidae 

 is considered to be due to convergence (Jaekel [237k]). A detailed 

 comparison of the two would be sure to yield interesting results. Fossil 



