; LAMELLIBRANCHIATA. 523 
Colpomya.] 
sulcus distinct, causing a flattening of the umbones and a sinus in the ventral mar- 
gin. Umbonal ridge prominent, strongly convex. Hinge plate straight, long, very 
thin posterior to the beaks, much heavier in front of them; beneath the beak of the 
right valve a tooth-like prominence which fits into a corresponding depression 
beneath the beak of the left valve; in front and beneath this depression in the 
left valve, a strong process projects obliquely downward, backward and toward the 
opposite valve, and is partly received in a socket that defines the anterior ‘side of 
the tooth in the right valve, while its lower end curves under that tooth. Muscular 
sears and pallial line apparently as in Modiolopsis, excepting that there is a small 
accessory scar in the hinge plate just behind the anterior adductor,as in Ischyrodonta. 
Type: Colpomya constricta n. sp. 
Fig. 41. Colpomya constricta Ulrich, top of Trenton group, Frankfort, Kentucky. a, right valve, 
showing the usual characters of the species; b, interior of a left valve; ¢, interior of an imperfect 
right valve. 
Colpomya evidently belongs to the Modiolopsidw with relations to Modiolopsis, Mo- 
diolodon and Orthodesma. In none of those genera, however, are the umbonal ridges 
and the mesial sulci quite such marked features, at any rate it would be rare, so that 
we may fairly regard their distinct development in shells of this family as indicative 
of Colpomya. When we come to internal characters all comparisons with the first 
and last of these genera may as well cease, since in both the hinge is practically 
toothless. In Modiolodon, however, we find cardinal teeth, but every one will admit 
that they are very different from those of the genus under consideration. There is 
nothing to represent the oblique process which projects under the tooth and hinge 
plate of the right valve, the teeth being approximately equal in the two valves of 
Modiolodon. 
The species to be placed ino this genus are not numerous and with two possible 
exceptions are all new. The exceptions are Modiolopsis milleri Ulrich, from the 
Cincinnati rocks, and M. faba Hall, said to be a Trenton and Hudson River species. 
The general expression of these shells is very much as in undoubted species of Col- 
pomya, but as their hinges are not yet known, their removal from Modiolopsis now 
would be of very doubtful advantage. Of four new species, C. demissa is a lower 
Trenton form, while the type of the genus and two other species occur in the 
upper Trenton of Kentucky. 
