LA MELLIBRANCHIA TA. 
31 
Right valve depressed, convex. Left valve unknown. 
Hinge-line straight, central, length one-half the length of the shell. 
Beak obtuse, rounded, not rising above the hinge-line; sides of the umbo 
straight. 
Ears small, sub-equal, narrow-triangular, separated from the sides of the 
umbo by sulci. Margin of posterior ear concave; extremity obtusely angular. 
Anterior ear defined by a deep, angular byssal sinus; margin convex; 
extremity rounded. 
The external markings of the test are not preserved. The cast of the 
interior is nearly smooth. Pallial line faintly impressed. At each side of 
the umbo there is a shallow, undefined impression, nearly parallel to the 
margin, extending to about the middle of the length of the shell. 
A specimen of the right valve has a height of 15 mm., length 17 mm., 
hinge-line 8 mm. 
This species approaches very nearly several forms referred to Crenipecten. 
More perfect material may show the hinge to be crenulate, which character 
would exclude it from Aviculopecten. 
Formation and locality. In conglomerate, referred to the Upper Chemung 
group, near Olean, Cattaraugus county, N. Y. 
Aviculopecten (Pterinopecten ?) invalidus. 
PLATE I, FIG. 18; and PLATE LXXX1I, FIG. 21. 
Ptenmopesten invalidus. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 1, fig-. 18. Jan., 1883. 
Shell small, rhomboidal; length and height nearly equal, moderately oblique; 
margins regularly rounded, full behind; the greatest posterior extension is 
below the middle. 
Left valve depressed-convex. Right valve unknown. 
Hinge-line nearly straight, length a little more than the length of the 
valve, and extending anteriorly. 
Beak obtuse, rounded, situated just anterior to the centre of the hinge-line, 
