RAJIFORMES 



165 



Family Ptychodontidak. A group of Cretaceous fish of which 

 little is known except the teeth. The dentition somewhat resembles 

 that of the Myliobatidae, there being longitudinal rows of teeth, of which 

 the middle row is the largest (Fig. 128). The large teeth are nearly 

 square in shape, and composed of thick vasodentine, with a ridged 

 surface (Owen [311a], A. S. Woodward [505a]). In the shape of the jaw 

 they approach the Trygonidae. 



Ptychodus, Ag. ; Cretaceous, Europe and N. America. 



Family Myliobatidae. As in the last family, the pectoral fins 

 generally meet in front of the skull ; but their anterior, preorbital 



Lower jaw of Ptychodus deeurrens, Ag. ; Cretaceous, Sussex. (After A. S. Woodward, 

 from Brit. Mus. Guide.) 



portions become separated off from the rest of the fin, so as to form a 

 special anterior cephalic fin, median in the Myliobatinae (Fig. 129, B). 

 In the Dicerobatinae this fin is developed as paired horn-like projections 

 (Fig. 129, C). 



The head, with its large protruding eyes, projects dorsally above the 

 level of the disk. The skeleton resembles that of the Trygonidae ; but 

 the ceratohyal is attached to the first branchial arch. The skin is smooth, 

 excepting for a strong serrated spine which generally lies behind a single 

 small dorsal fin, near the base of the elongated whip-like tail (Fig. 129). 



Sub -Family 1. Myliobatinae. With a median 'cephalic fin,' 

 quite continuous with the pectorals in Promyliobatis. The teeth form a 

 flat grinding pavement (Fig. 130), of which the central row becomes 



