SUB-CLASS I 



SELACHI1 



43 



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



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

 which are interrupted at the sides of the head, but 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 dost 

 pari an at in both jaws. Skin naked. Cretaceous (1), 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 crown, which is 

 transversely or radially ribbed, and sharply 

 separated by a constriction from the smaller, 



Fig. 85. 



odus^olygyrus, Ag. Tooth, oral and posterior views. 

 tat. size. Greensand ; Regensburg. 



Fir,. 86. 



Ptychodus decurrens, A.u. Diagram of 

 arrangement of teeth in upper (A), and 



lower (II) jaws, much reduced. I. own- 

 Chalk ; England (after Smith Wood- 

 ward). 



smooth root.l 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 Myliobatis, lateral aspect. 



Fig. 88. 



.Jaws of Myliobatis 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. 



