|g ; PALAEONTOLOGY OP NEW-YORK. 



BisciNA GHAMiis ( V'anaxem). 



Geological formation and locality. In the compact arenaceous shales, at Caze- 

 novia, and at Pratt's falls in Onondaga county ; and a single fragment has been 

 found near Moscow, Geneaee county. 



Disciiia rauilalli ( n. s.). 



PLATE n. 



Ventkal valve circular or nearly circular, gently concave within the 

 margin : foramen large, marked by a broadly oval depression on the 

 exterior surface, which reaches half the distance from the apex to the 

 margin. 



Surface marked by strong rounded concentric ridges with sharp depres- 

 sions between, and sometimes with finer concentric striaB upon the 

 coarser ones ; all of them crossed, on the posterior margin of the shell, 

 by fine radiating striae or vascular impressions, which enter into the 

 substance of the shell. On the anterior half of the shell, the concentric 

 striae become partially obsolete. From a constricted ridge, which exter- 

 nally marks the place of a median septum in the muscular impression, 

 there diverge strong rounded radiating ridges, separated by narrow 

 abrupt depressions which extend nearly or quite to the margin of the 

 valve. 



The single specimen known is a partially exfoliated exterior surface of a ven- 

 tral valve : the margin of the anterior portion is somewhat broken and worn 

 away. Certain appearances indicate that the dorsal valve lies beneath this ventral 

 valve, imbedded in the matrix, and the two pressed closely together. 



This species is a large and remarkable form ; the transverse diameter is nearly 

 three inches, and from the apex to the posterior margin is one inch and a half. 



