Oolpomya.] LAMELLIBRANCHIATA. 523 



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 

 scars 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 conxtricta Ulrich, top of Trenton group, Frankfort, Kentucky, a, right valve, 

 showing the usual characters of the species; 6, interior of a left valve; c, interior of an imperfect 

 right valve. 



Colpomya evidently belongs to the Modiolopsidce 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. 



