i6o 



SELACHII 



undulating grinding pavement (Fig. 123). In the Jurassic species the 

 anterior vertebrae remain free, or are less fused than in the modern species. 

 No oro-nasal grooves, except in Trygonorhina, which with its confluent 

 nostrils is so skate-like that it is difficult to say whether it should be 

 put here or in the family Rajidae. 



fihinobatus, Black ; tropical and temperate seas ; to Upper Jurassic, 

 Europe, and Cretaceous, Syria. Rhynchobatus, M. and H. (Fig. 122); 

 Indian seas and Pacific. Trygonorhina, M. and H., Platyrhinoidia, Gar. ; 

 Pacific. 



Family PRISTIDAE. Tho body is shark -like and little depressed, 

 with a well-developed caudal region and slight longitudinal keel (Fig. 

 116). The first dorsal is almost on a level with the pelvic. The pectorals 

 are large, but do, not reach the skull, although passing forwards into u 

 fold on the head. The teeth are numerous, small, and blunt. 



Fia. 123. 

 Dentition of Xhynchobatiis sp. (After Gunther.) 



In the modern Saw-fish, Pristis, the rostrum is drawn out into a long 

 flattened blade bearing a single series of large teeth along its lateral edge 

 (Fig. 116). Each of these modified denticles has a persistently growing 

 base deeply embedded in a socket in the hard calcified cartilage of the 

 rostrum. The Cretaceous genus Sclerorhynckus (A. S. Woodward [503]) 

 shows a more primitive condition, in which the rostral teeth are small 

 and lie in the skin. Jaekel [238] considers that the shark- like 

 shape of the body has been secondarily acquired, and that the Pristidae 

 are derived from the Rhinobatidae. In no other way can we account 

 for the condition of the pectoral fins, branchial slits, and vertebral 

 column, characters acquired apparently in adaptation to a mode of life 

 which the Pristidae have abandoned. 



SclerorhyncJms, A. S. W. ; Cretaceous, Syria. Propristis, Davis ; 

 Eocene, Egypt. Pristis, Lath. ; tropical and subtropical seas ; Eocene, 

 Europe and N. America. 



Family RAJIDAE. The adaptation to bottom living is carried to 



