978 PALEONTOLOGY OF NEW- YORK. 



middle towards the beaks ; a somewhat defined ridge extending from 

 about the middle of the shell to the umbo, which is prominent and 

 elevated above the hinge-line ; beak incurved : anterior side sub- 

 auriculate, with a shallow undefined sinus separating this portion from 

 the body of the shell, the sinuosity being perceptible in the outline of 

 the margin ; posterior cardinal slope moderately expanded, but not 

 alate. 

 Sdbface marked by concentric lamellose striae. 



In this and the preceding species, the shell has been partially worn or exfoliated, 

 so that the original surface characters are not fully preserved : the form of the 

 shells, however, as shown in the figures, is sufficiently distinctive. 



 Fig. 5. The left valve of this species. 



Geological position and locality. In the shaly limestone of the central portion of 

 the Lower Helderberg group : Schoharie county. 



meganibonia cordifonuis ( n. s.). 



Plate L. Fia. 6 a, b. 



Shell cordiform, extremely gibbous from the base upwards to the um- 

 bones, which are elevated and incurved, not compressed at the basal 

 or lateral margins ; anterior cardinal extremity slightly auriculate, the 

 prominence covering the muscular impression small. 



Surface marked by concentric lamellose striae. 



The only specimen which I have seen is apparently somewhat vertically com- 

 pressed, and the posterior cardinal slope is worn off from exposure upon the surface 

 of the stratum. If compressed in the opposite direction, it would have somewhat 

 the shape of M. lata ( fig. 4 ) ; but it is less oblique, and the anterior extension 

 appears never to have been as great. 



Fig. 6 a. Profile view from the anterior side of the shell. 



Fig. 6 6. View of the right valve, having the posterior cardinal extension abraded. 



Geological position and locality. In the central portion of the Lower Helderberg 

 group : Schoharie county. 



