SUB-CLASS I 



SELACHII 



43 



Family 12. Myliobatidae. Miiller and Henle. Eagle-rays. 



Trunk much depressed, forming a broad disk with the very large ijedoral fins, 

 loliich are interrupted at the sides of the head, hut reappear as one or a pair of small 

 cephalic fins at the extremity of the snout. Tail very slender, with one or several 

 flattened spines behind the single dorsal fin. Teeth rather large, forming a close 

 pavement in both jaws. Skin naked. Cretaceous (?), Tertiary and Recent. 



Most of the surviving genera of this family are represented in the Tertiary 

 by isolated teeth and spines, or by the complete dentition. 



1 Ptychodus, Ag. (Aulodus, Dixon), (Figs. 85, 86). Known only by the 

 dentition and vertebrae, and hence of uncertain systematic position. Teeth 

 quadrangular, with a raised crowai, which is 

 transversely or radially ribbed, and sharply 

 separated by a constriction from the smaller. 



Fig. 85. 



Ptychodus -polygyrus, Ag. Tooth, oral aud posterior views 

 nat. size. Greeusand ; Regensbiirg. 



Fro. 86. 



Ftyrhodus decurrens, Ag. Diagram of 

 arrangement of teeth in upper {A), and 

 lower (U) jaw.s, much reduced. Lower 

 Chalk ; England (after Smith Wood- 

 ward). 



smooth root. The teeth are solid, and the ganodentine investing the crown 

 is thick. They are arranged in about thirteen antero-posterior series (Fig. 

 86). In one jaw, presumably lower, the median series of teeth is the largest, 

 and the lateral rows are dis- 

 posed symmetrically, diminish- 

 ing in size outwards. In the 

 opposing jaw the median series 

 is very small, and the first 



Fig. 87. 

 Jaws of Recent Myliohatis, lateral aspect. 



Jaws of Myliohatis aquila, Cuv. Anterior aspect. 

 Mediterranean (after Agassiz). 



Recent'; 



lateral row on each side large, with the outer lateral series diminishing. 

 Common in the Chalk of Europe and North America, rarer in the Greensand. 



