LAMELLIBRANCHIATA OF THE LOWER MARLS. 89 



regularly rounded; posterior end elongate, produced below, the posterior 

 margin very oblique, so as to make the postero-basal angle quite ai-ute. 

 Hinge-plate very narrow, marked by oblique transverse teeth, the anterior 

 end having ten or twelve directed inward below, and the posterior a some- 

 what larger number pointed in the opposite direction. Muscular impression 

 large, the posterior one bordered by a slightly elevated lamella on the an- 

 terior margin. Surface of the shell marked by radiating lines, strongest on 

 the anterior end and faintest on the middle of the valve ; also by concentric 

 lines which cross them and form slight pustules at the junctions. 



The specimens used are Mr. Conrad's types, a right and a left valve, 

 and an internal cast of a right valve. They have much the look of j^oung 

 shells, and may prove to be such when the locality is further examined 

 The ligamental area is very narrow, and on the small specimens scarcely 

 perceptible with an ordinary lens. The hinge-plate is reduced to almost a 

 line in the middle, and the teeth and sockets are reduced to a minimum size, 

 while a little po.sterior to the central part there is a .short space destitute of 

 either teeth or sockets. The ridge bordering the posterior muscular im- 

 pression is very slight and not at all thickened, so that in the cast the gash 

 marking its place is scarcely perceptible. 



Formation and locality. — In the micaceous clays of the Lower Marl Beds 

 at Haddonfield, New Jersey. From the collection of the Academy of Nat- 

 ural Sciences of Philadelphia. 



Trigonarca transversa. 



Plate XII, Figs. 13-16. 



Cucullwa transversa Gabb. P. A. N. Sci., 1861, p. 326. 

 C. transversalis (Gabb). Meek, Check-list. p. 8. 



Idonearca transversa (Gabb). Meek, Geol. Surv. N. J., 1868, p. 725. 



Shell of medium size, transversely ovate, nearly twice as long as high, 

 exclusive of the large, prominent, slightly incurved and distant beaks, which 

 are situated at about the anterior third of the length of the valve. Valves 

 very ventricose, with a long hinge-line reaching to near the anterior end 

 jvnd nearlv two-thirds as long' as the greatest length of the shell Anterior 

 end not extending beyond the hinge-line, but curving with a broad sweep 

 into the basal line below, which is almost regularly but broadly curved; 



