470 PALEONTOLOGY OF NEW YORK. 



from the beaks to the base of the shell as a marked depression and producing 

 a slight sinuosity in the margin. Umbonal slope prominent, often distinctly 

 angular, extending to the post-inferior extremity. Post-cardinal slope gently 

 concave, often marked by a slight fold along the middle or by two or more 

 radii. 



Surface marked by fine concentric striae, and by strong radii diverging 

 from the beak, usually marking that portion of the shell between the cinc- 

 ture and the umbonal ridge, sometimes covering the entire surface 



Four specimens measure respectively 19, 22, 28 and 36 mm. in length, and 

 10, 11, 13 and 18 mm. in height. The largest specimen observed has a length 

 of 43 mm. 



The majority of the specimens are of young individuals and present a con- 

 siderable variety of iispect, chiefly, however, in the development of the radii 

 upon the surface, of which there are sometimes not more thfin six or seven 

 upon the body of the shell, while in other examples these radii cover the entire 

 surface. The specimens also vary considerably in the form of the posterior 

 extremity, which is sometimes narrowly rounded, often obliquely or vertically 

 truncjite. An examination of a sufficient number of specimens shows no 

 rea.son for retaining the species heretofore indicated as P. trnncata and P. ornata. 



Formations and localities. In the shales of the Hamilton group in Schoharie 

 county ; near Fabius and at Pratt's falls in Onondaga county, and in some con- 

 cretionary layers at Eighteen-mile creek, Lake Erie shore, N. Y. It is found 

 in the same horizon near Cumberland, Md. ; also in the lower part of the Che- 

 mung group at Ithaca, N. Y. 



Pholadella parallela. 



I'LATK i.xxvni, KIUS. -la-H. 



OrammyxM pnrajleta. Hall. I'pf'lim. Notico LnmcHiI>ranchiatii. 2, p. 59. 1H70. 



P/iolatUUa " " Pal. N. v., vol. V, pt. 1. Plalos luul Kxplaiuitions : PI. 78, %s. 22-24. 1883. 



Shem. small, .short, ovate-cuneate; length less than twice the height; basal 

 margin gently curving or nearly straight. Posterior extremity obliquely 



