52 



PISCES. 



of Bavaria, where complete skeletons occur (fig. 39) ; the only 

 noteworthy difference between the Jurassic and Recent species 

 hitherto observed being, that the vertebral centra of the former 

 exhibit fewer peripheral calcified rings than the latter. The 

 Pristiophoridae certainly range from the Upper Cretaceous 

 onwards, an undescribed specimen from the Chalk of Mount 

 Lebanon being preserved in the Museum of the Syrian Pro- 

 testant College, Beyrout. The Pristidae are of more interest, 

 there being some evidence that their characteristically-toothed 

 rostrum has only been acquired during Tertiary times. The 

 earliest genus ascribed to this family is Sclerorhynclius (fig. 40) 

 from the Upper Chalk of Mount Lebanon ; and in this fish the 

 rostral teeth are comparatively small, while the bases of some 



FIG. 41. 



Ptyr.hodus polygyrus ; tooth, coronal aspect and transverse section of crown, 

 about nat. size. U. Cretaceous (English Chalk). 



are not even in contact with the supporting cartilages. The 

 Eocene Propristis, known only by fragments of rostrum, is 

 described as having the rostral teeth free, not embedded in 

 cartilage. The typical Pristis-rostrum also dates from the 

 Eocene. The earliest Rhinobatidae are Upper Jurassic, and 

 the beautifully preserved skeletons from the Lithographic Stone 

 scarcely differ from that of the existing Rhinobatus. Before 

 the close of the Cretaceous period the Rhinobatidae, Rajidse, 

 and Trygonida? do not appear to have been so clearly differen- 



