DISCING OF THE HAMILTON GROUP.. 
17 
Geological formation and locality. In the Marcellus slate, near Bridgewater ; 
and in the shales of the Hamilton group, on Canandaigua lake. 
Discina grandis. 
PLATES I & II. 
Orbicula grandis* : Vanuxem, Geological Report Third District, 1842, p. 152-3, f. 4. 
General form broadly and transversely elliptical, plano-convex or 
concavo-convex. Dorsal valve sometimes extremely elevated; apex 
subcentral, a little on one side of the transverse axis. Ventral valve 
usually moderately concave ; foramen reaching from the centre or near 
the centre towards one side, but varying somewhat in different indivi¬ 
duals. 
Surface marked by fine concentric striae, crowded near the centre, and 
more distant and sharply elevated towards the margin. 
This species is recognized by its large size, and in the ventral valve by the 
direction of the foramen being in the shorter diameter of the shell. This valve 
is somewhat unequally concave, and, on the side of the foramen, often a little 
convex. The dorsal valves of two specimens, which appear to be of this species 
(one of them with ventral valve attached), are very different in their degree of 
convexity, and may prove distinct. 
In different specimens, this species has a transverse diameter of one inch and 
a quarter to one inch and five-eighths, with a longitudinal diameter of one inch 
and an eighth to one inch and a quarter. 
The original figure of Vanuxem, given below, is of the ventral valve. A single 
specimen retaining the two valves in connexion, has been observed ; while a 
separate valve of this, or an allied species, has been found in the same associa¬ 
tion. So far as the collections yet obtained furnish evidence, this species is.very 
rare; and but a single fragment (and this may be questioned ) has been found 
to the west of Cayuga lake. 
* This name was overlooked at the time of describing the species in the Oriskany sandstone, and will be 
superseded for that species, the one under consideration having precedence. I propose for the Oriskany 
form the name Discina ampla. 
[ Palaeontology IV.] 3 
