LAMELLIBEANCHIATA OF THE LOWER MARLS. 95 



Genus IDONEARCA Conrad. 

 (P. A. N. Sci. Phil., Vol. XI Y, p. 28 J 5 ib., p. 51, 1872.) 



Idonearca Tippana. 

 Plate XII, Figs. 19-21. 



jucullcea Tippana^ Conrad. Jour. A. N. Sci., Phil., 2d ser.. Vol. Ill, p. 329, PL XXXV, 

 Fig. 1. Gabb, Synop., p. 118. Meek, Check list, p. 8. 



Idonearca capax, Conrad. Proc. A. N. Sci., Phil., 1872, p. 54, PI. II, Fig. 2. 

 Kot Idonearca capax, Conrad. Jour. A. N. Sci., Phil., new series. Vol. Ill, p. 328, PL 

 XXXV, Fig. 2. 



Shell of medium size, inequilateral and obliquely triangular, with deep 

 valves and a very angular umbonal ridge and abrupt posterior slope of 

 but little width; beak only moderately prominent; incurved and approxi- 

 mate. Cardinal area only moderate, compared with those of the other species 

 associated with it. Hinge- line about half as long as the shell or a little 

 more. Anterior end broadly rounding into the basal margin which is nearly 

 straight in its posterior part, and the postero-basal angle strongly marked. 

 Surface, so far as can be judged from the cast, marked only by concentric 

 lines. Muscular ridge, as indicated by the gash left in the cast, large and 

 strong, reaching nearly to the margin of the valve, and situated nearly 

 midway between the umbonal ridge and the cardinal border. Anterior 

 muscular scar not distinctly traceable. Hinge teeth and features not plainly 

 seen. Edge of the valves on the interior surface crenulate. 



The New Jersey casts agree very closely indeed with the figure of 

 /. Tippana given by Mr. Conrad, in form, size, and general character, 

 although the strong language used in his description would somewhat mis- 

 lead in the absence of specimens. With examples of the shell also from Ala- 

 bama they correspond very closely. It is the most distinctly triangular 

 form occurring in the New Jersey formations, which form, together with the 

 sharply angular umbonal ridge and abrupt postero-cardinal slope, will 

 serve to distinguish it from any other form exteriorly. Casts of the interior, 

 however, made from one of the Alabama specimens, is so very similar to 

 the smaller casts of I. vulgaris, that it would require a critical eye indeed 

 to detect the differences. The hinge of the Alabama specimen above re- 

 ferred to is very broad and the shell very thick; the teeth are strong and 

 few in number, and do not differ very materially from those of /. vulgaris 



