LAMELLIBRANCHIA TA. 
481 
The specimen described is incomplete at the posterior extremity, and 
had originally a length of about 42 mm. and a height of 5 mm. 
This species, in its narrow and extremely elongate form, resembles 0. cari- 
nata, but differs in its ensiform, arcuate character, and the absence of promi¬ 
nent carinse on the posterior slope. 
j Formation and locality. In the shales of the Hamilton group, in Schoharie 
county, N. Y. 
Orthonota rigida, n. sp. 
PLATE LXXX, FIG. 6. 
In part SanguinQlites undatus, Hall. Pal. N. ¥., vol. v, pt. 1. Plates and Explanations: PI. SO. 
figs. 5, 6. 1.SS3. 
Shell small, trapezoidal; length more than twice the greatest height; basal 
margin straight in the posterior half, gradually curving upward toward the 
anterior end. Posterior extremity nearly vertically truncate. Cardinal line 
straight, extending for three-fourths the length of the shell. Anterior end 
scarcely declining from the beaks and rounded beloiv. 
Valves depressed-convex below, convex in the umbonal region. 
Beaks at about the anterior fourth, small, appressed, scarcely rising above 
the hinge-line. Umbonal slope angular, extending to the post-inferior 
extremity. Post-cardinal slope wide, depressed, marked along the centre by 
a depression, which is defined in the anterior part and obscure toward the 
posterior. 
Surface marked by tine concentric striae, and by regular continuous con¬ 
centric undulations, which are less conspicuous toward the basal and cardi¬ 
nal margins. 
The specimen described has a length of 28 mm. and a height of 12 mm. at 
the posterior end. 
This species differs from 0. undulata, in its shorter and broader form, the 
umbonal slope is more distinctly angular, and is not limited by a groove below ; 
the post-cardinal slope is not so strongly marked by the undulations and has no 
distinct fold extending along the middle of its length. 
Formation and locality. In the Chemung group, near Elmira, N. Y. 
