PELECYPODA—PRIONODESMA CEA . 



395 



68. C. equilatera Hall. Siluric. 



Elliptical ; anterior and posterior ends nearly equal. A groove 

 extends from the beak nearly to the posterior extremity. Dimen- 

 sions i|x | inches. 



Cobleskill : New York. 



XXVI. Nucula Lamarck. 



Pearly, small, equivalve, varying in outline from trigonal to 

 nearly circular or transversely elliptical. Beaks often subcentral, 

 usually posterior to the middle and turned backward. Surface 

 marked by concentric striae. Hinge with a triangular resilifer 

 beneath the beaks and a series of small transverse teeth on each 

 side. Two subequal adductor muscle impressions in each valve. 

 Pallial line simple. Interior margins often crenulate. (Probably 

 a polyphyletic group.) Ordovicic-Recent. 



69. N. lirata (Conrad). (Fig. 503, d.) Devonic. 



Fig. 503. a, b, Nucula hottghtoni ; c y A\ randalli ; d, N. lirata ; e, N. bellistriata ; 

 /, N. corbuliformis, (N. Y. Surv. ) 



Gibbous. Concentric undulations strong and subangular. 

 Hamilton : New York, Pennsylvania, Ontario. Onondaga : 

 Indiana. 



70. N. randalli Hall. (Fig. 503, c.) Devonic.. 

 Valves ventricose, their depth conjoined equalling their height. 



Striae fine and thread-like but interrupted, especially toward the 

 front, by growth varices. 

 Hamilton : New York, etc. 



71. N. bellistriata (Conrad). (Fig. 503,^.) Devonic. 

 Differs from N. randalli in its broadly ovate outline, less 



gibbous valves, and finer striae, usually without varices of growth 

 Hamilton : New York, etc. 



