ELASMCXBRANCHII. 99 



Differs from Myliobatis in having- the teeth uniserial, very 

 broad, and no small lateral ones. Upper dental lamina straight 

 and lower lamina projecting" beyond upper, curved. Free hind 

 edge of nasal valve deeply emarginated. 



A single existing species in tropical seas, and 13 extinct species 

 have been described, the latter only known from dental plates. 



Aetobatus pErspicuus (Leidy). 



Aetobatis perspicuus Leidy, Proc. Acad. Nat. Sci. Phila., 1855, p. 396. Mon- 

 mouth Co., N. J. Eocene? 



JEtobatis perspicuus Leidy, Journ. Acad. Nat. Sci. Phila., (2) VIII, 1877, p. 

 2+4, PI. 31, fig. 13 (type). 



Median tooth of dental plate well arched or angxilar, turning 

 abruptly upward at end of tooth. Enamel surface rather broad, 

 flat, and end only slightly rounded. Greatest width or vertical 

 diameter of enameled surface usually uniform, and about one- 

 seventh in entire horizontal diameter. Anterior edge of tooth 

 angularly convex greater part of its extent, feebly deflected for- 



Fig. 52. — Aetobatus perspicuus (Leidy). (From Leidy.) 



ward laterally, and at end presents projection adapted to cres- 

 centoid depression or socket of contiguous tooth. Posterior edge 

 presents a narrow flang'e along greater portion of its length, and 

 crescentoid socket at end of tooth adapted to receive projecting 

 border of contiguous tooth. Root with laminae projecting poste- 

 riorily about space equal to one-half vertical diameter of enameled 

 surface, and all directed obliquely towards median axis of tooth. 

 Length ( width) about 60 mm. (From Leidy.) 



Formation and locality. Only the type known described above, 

 from the Eocene of Monmouth County (J. L. Burtt), originally 

 presented to the Academy, but which I have not seen. 



