430 CRETACEOUS PALEONTOLOGY. 



Description. Shell oblique, winged in front and behind, the 

 hinge-line straight with the beaks in front of the middle. Both 

 valves rather strongly convex, but the left a little more so than 

 the right. Posterior wing compressed, of moderate length, 

 pointed behind, its posterior margin concave; anterior wing nar- 

 rower, pointed in front, less compressed than the other, its free 

 margin nearly straight or slightly concave ; in the right valve it is 

 separated from the body oi the shell by a narrow and shallow 

 sulcus which extends from the anterior side of the beak down- 

 ward and usually a little obliquely backward to the antero-ventral 

 margin; just in front of the marginal extremity of this sulcus 

 the surface is slightly bulged so as to leave a byssal opening be- 

 tween the valves. The antero-ventral margin slopes obliquely 

 backward from the anterior extremity of the hinge-line; it is 

 slightly concave to the base of the anterior wing beyond which 

 point it becomes slightly convex, curving more and more below 

 into the rounded postero-basal margin; the posterior margin 

 oblique below and sinuate above. Surface of the shell marked 

 only by concentric lines of growth which are inconspicuous on 

 the internal casts, 



The dimensions of a large specimen are: length from the an- 

 terior extremity of the hinge-line to the postero-basal margin, 

 5 1 mm. ; length of hinge-line, 37 mm. ; distance of beak from the 

 anterior extremity of hinge-line, 12 mm.; convexity of right 

 valve, 10 mm. 



Remarks.- This species has always been considered one of the 

 rare forms in the New Jersey Cretaceous faunas. Whitfield saw 

 only one individual from near Keyport, and this specimen, aside 

 from the type which is stated to be from Delaware, is the only 

 one on record. In the recent collections of the Survey, the species 

 occurs in abundance in the nodules from the Cliffwood clays, 

 and less commonly from the summit of the Wenonah sand. 

 Whitfield's specimen from "at or near Keyport" most probably 

 was collected from the Cliffwood nodules at Cliffwood Point. 

 Different individuals of the species show considerable variation 

 in the obliquity of the shell, and also in the extension of the 

 posterior wing, although the variation in this latter character 



