LAMELLIBRANCHIATA OF THE LOWER MARLS. 121 



The species differs from any described form from the American Cre- 

 taceous in the ventricose valves, coupled with the large, nearly central, erect 

 beaks and the evenly balanced transverse form. 



Formation and locality, — In the Lower Marls at Crosswicks, New Jersey. 

 The specimen figured is the type used by Mr. Conrad, and is from the col- 

 lection of the Academy of Natural Sciences of Philadelphia, where it is 

 accompanied by a much smaller individual of much the same form. 



CrassateUa subplana. 



Plate XVIII, Figs. 14-16. 



CrassateUa subplana Conrad. J. A. :^. Sci., Vol. II, new s^er., p. 247, PL XXIV, Fig. 9. 

 Gabb, Synop., p. 113. Meek, Check-list, p. 11. 



Shell of about a medium size, somewhat subtriangular in outhne; nearly 

 as- high as long, with compressed valves, subangular along the umbonal 

 ridge, and with small, appressed, pointed, and erect beaks, which are situ- 

 ated a little in advance of the middle of the length. Anterior end of the shell 

 rounded, the greatest length at or just below the middle of the height; pos- 

 terior end subtruncate below and obliquely sloping to the beak above; basal 

 Hue fullest anterior to the middle and just perceptibly sinuate between that 

 point and the posterior umbonal extremity. Surface of the shell marked 

 by more or less irregular elevated concentric ridges, which are flattened on 

 the tops and separated by equally wide, concave, or flattened interspaces. 

 Substance of the shell thick. The interior of the valves is marked with 

 comparatively deep but not large muscular scars, a wide hinge-plate, and 

 strong, well-marked teeth; margin crenulate. The internal casts are dis- 

 tinguished by their proportional height and compressed valves. 



This species is the most elevated form of CrassateUa found in the New 

 Jersey Cretaceous, and may be distinguished by this feature from any of the 

 others. In the younger stages of growth the shells are much more nearly 

 quadrangular in outline and sometimes more ventricose than the specimens 

 figured. In this condition they very closely resemble (7. curta Conrad 

 from the Miocene of Virginia. 



