DISCINJE OF THE HAMILTON GROUP. IT 



Geological formation and locality. In the Marcellus slate, near Bridgewater ; 

 and in the shales of the Hamilton group, on Canandaigua lake. 



Disciiia grandis. 



PLATES I & IL 

 Orbicula grandW : 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 strias, 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 overloolccd at the time of describing the species in the Orisljany sandstone, and will bo 

 superseded for that species, the one under consideration having precedence. I propose for the Orislcany 

 form the name Ditcina ampla. 



[ Paljeontoloqy IV.] 8 



