18 
; PALAEONTOLOGY OF NEW-YORK. 
Discina grandis (Vanuxem). 
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, Genesee county. 
Discina randalli (n. s.). 
PLATE II. 
Ventral 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 striae 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 sheik 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, 
