166 
PALAEONTOLOGY OF NEW YORK. 
of the right valve is less defined than in the left, and the byssal sinus is a 
little deeper. 
Test thin; specimens partially exfoliated, or occurring in the condition of 
casts, show sharply elevated, lamellose, concentric striae, which are very dis¬ 
tinct on the wings, and crowded and somewhat fasciculate on the anterior 
side and ear. No intermediate finer striae have been observed. In well- 
preserved specimens the sharp lamellae have undulating margins. 
Interior unknown. Ligamental area marked by a single groove. 
A large specimen has a length of 43 mm., height 50 mm., hinge-line 37 mm. 
Another example has a length of 32 mm., height 33 mm., hinge-line 30 mm. 
In the younger shells the height is proportionally somewhat less than in 
the older ones. 
This species, compared with the preceding, has a greater height of body and 
shorter hinge-line, and differs in the conspicuous, sharp, concentric lamellae. 
In this species the concentric striae do not produce the folds and undulations 
which are characteristic of L. Rafinesquii and L. Greeni. 
Formation and localities. In shales of the Hamilton group at Pratt’s falls, 
Onondaga county, and in the coarser beds at Schoharie, N. Y. 
Leiopteria Mitchelli. 
PLATE XX, FIG. S ; AND PLATE, LXXXVIII, FIG. 26. 
Leiopteria Mitchelli, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. £0, fig. 8. Jan., 1883. 
Shell large, sub-rhomboidal; body ovate, moderately oblique; height greater 
than the length; anterior margin nearly vertical from the ear for about half 
the height, then broadly curving to the post-basal side, thence more abruptly 
bending, making the posterior end slightly extended. 
Left valve gibbous on the umbo, sloping from highly convex above to 
moderately convex below. Right valve unknown. 
Hinge-line straight, equal to the length of the valve. 
Beak anterior, acute, inclined forward, prominent. Umbonal region prom¬ 
inent, subtending an acute angle. 
