Descriptions of N'eir Genera and Species of Fossils. If) 



Orthis? sectostriata, 11. sp. (Plate VII., figs. 11, \la and 116.) 



[Ely.— Seco, to cut, to divide-] 



Shell attaiuiug medium size, sub-circular ; A^alves nearly equally 

 convex. Dorsal valve convex, its greatest prominence being near the 

 middle of the valve; a slight mesial ridge runs from the umbo to the 

 untorior margin; beak short and incurved. Ventral valve with great- 

 est convexity a little posterior to the middle; mesial sinus but slightly 

 defined; beak ver^' prominent, obtusely pointed, and quite strongl3^ in- 

 curved upon that of the other valve. Surface ornamented with from 

 thirty to thirtj'-five fine, even, radiating striae, all of which bifurcate 

 once near the center of the valves, making the number at the 

 anterior margin about seventy. On well preserved specimens, when 

 viewed through a magnifier, the surface exhibits veiy fine and crowded 

 concentric lines. 



Length of a medium sized specimen. 



The generic affinities of this specis can not, at present, be positively 

 determined, since its interior is as yet unknown. It is possible that it 

 should be placed into the genus Zygospira, Hall, as it externally much 

 resembles Zygo":pira headi, Billings. From O. ella. Hall, it is dis- 

 tinguished by its circular outline, much finer and bifurcated striae; the 

 beak of the ventral valve is much more incurved upon that of the dorsal 

 valve, than is the case in O. ella.. 



Formation and locality: the species is not common, and occurs in the 

 Hudson River Group, on the hills back of Cincinnati, Ohio, at an eleva- 

 tion of about three hundred and seventy-five feet above low watermark 

 in the Ohio River. 



Collector: the' specimen figured was found by Mr. Stanage, A 

 number of specimens have been found by other collectors. 



Genus Lept^na (Dalman). 

 LEPT^iNA plicatella, u. sp, (Plate VII., figs. 12, 12fl, 126 and 12c'.) 



[Eiy .—PUcatella , a small plait or fold.] 



Shell yery small, semi-oval, approaching semi-circular, concavo-con- 

 vex ; hinge line sometimes only as long as the greatest breadth 

 of the valves, but generally its length is greater than the breadth of 

 the shell; lateral extremities varying from acutely angular to rectang- 

 ular, and not reflexed ; anterior and lateral margins forming together 

 nearly a regular semi-circular cui've. 



Ventral valve rather strongly convex, being almost evenly arched 

 along the middle from the beak to the front; beak very small, scarcely 

 distinct from the cardinal margin; area moderately developed, twice as 

 high as that of the other valve, inclined slightly backward; foramen 



