1906.] 



SELACHIAN FISHES. 



755 



In all the members of this family the teeth are small and 

 obtuse. There is a more or less distinct longitudinal fold on each 

 side of the tail. The differences in the shape of the body and the 

 development of the pectoral fins are considerable, but from 

 anatomical considerations there can be no question that we are 

 here dealing with a very natural group. DiscobaUis, in which 

 the pectoral fins extend as far forward as in liaicc, resembles 

 Rhinohatus in the arrangement of the cartilages of the dorsal fin 

 and in the shape of the ventrals, and should not be placed in the 

 Raiidse. The number of radials directly attached to Lhe pectoral 

 arch varies from none in Sclerorhynchus and one in Pristis to eight 

 in Trygonorhina. 



Family 2. Raiid^. 



Prseorbital cartilages articiilated to the propterygia. Usually 

 two small dorsal fins near the extremity of the tail ; caudal fin 



Text-fig. 124. 



Skeleton of the dorsal fin of Torpedo (A), Rhinohatus (B), Myliohatis (C), 

 and Psammobatis (D). 



b, basals ; r, radials. 



small or absent ; pectorals extending to the snout, sometimes 

 confluent anteriorly ; pelvics notched. Basalia of the dorsal fin 

 directed obliquely upwards and backwards, anteriorly imbedded 

 in the body, elsewhere connected b}'- a membrane to the back of 

 the tail ; radialia nearly reaching the free edge of the fin, some 



