76 ■ SYSTEMATIC PALEONTOLOGY 



nine times broader than long, and with nearly straight transverse 

 sutures. 



The specimen upon which the above diagnosis is based was recog- 

 nized by Leidy as belonging probably to a distinct species, its most 

 obvious characteristic being the nearly straight course of the transverse 

 sutures. The latter are not curved posteriorly at the sides nor in the 

 middle, as in M. gigas, nor is there a median longitudinal depression, 

 as in that and various other species. The median teeth are also rela- 

 tively longer than in the lower dentition of M. gigas, but the cross- 

 section is much the same in both. 



In the specimen under consideration, which appears to be unique, 

 the superficial layer of gano-dentine has been entirely removed, so that 

 the triturating surface presents a punctate appearance where the num- 

 erous nutrient tubules are exposed. Indications of wear are very con- 

 spicuous on the three anterior teeth, as shown in the figure. Owing 

 to the great amount of attrition which the median teeth have under- 

 gone, their thickness is nowhere more than 1 cm. The width of the 

 second median tooth counting from the front may be safely estimated 

 at 5 cm., and its length 0.9 cm. No significance is to be attached to 

 the slight irregularities in the course of two of the transverse sutures. 



Occurrence. — Calvert Formation. Charles county near the Patux- 

 ent river. 



Collection. — Philadelphia Academy of ^Natural Sciences. 



Genus AETOBATUS Blainville. 



Aetobatis aecuatus Agassiz. 

 Plate XXIX, Fig. 5. 



Aetobatis arcuatus Agsissiz, 1843, Poiss. Foss., vol. iii, p. 327. 



Aetohatis arcuatns Eastman, 1901, Md. Geol. Survey, Eocene, p. 103, pL xiii, figs. 

 3a, 3b, 8. 



Aetobatis arcuaUis Hay, 1903, Bull. 179, U. S. Geol. Survey, p. 331. 



This species is tolerably abundant in various Miocene localities of 

 Maryland, Virginia, North Carolina and Xew Jersey, but the teeth 

 invariably occur singly in the detached condition, and are more or 

 less water-worn or otherwise abraded. An examination of the type- 

 specimens of Cope's A. profundus, described from the Miocene of 



