470 
PALAEONTOLOGY 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 aspect, chiefly, however, in the development of the radii 
upon the surface, of which there are sometimes not more than 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 
truncate. An examination of a sufficient number of specimens shows no 
reason for retaining the species heretofore indicated as P. truncata 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. 
PlIOLADELLA PARALLELA. 
FI. ATE LXXVIII, FIGS. 22-24. 
Grammysia varallela, Hall. Prelim. Notice Lamellibranchiata, 2, p. 59. 1870. 
Plioladella “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 7S, figs. 22-24. 1883. 
Shell small, short, ovate-cuneate; length less than twice the height; basal 
margin gently curving or nearly straight. Posterior extremity obliquely 
