LAMELUBRAXCWATA. 31 



Right valve depressed, convex. Left valve unknown. 

 Hinge-line straight, central, length one-half the length of the Bhell. 



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. Pallia! 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 Glean, Cattaraugus county, N. Y. 



Aviculopecten (Pterinopecten ?) invalidus. 



PLATE I. KM. 18; LKD PLATK I.WMI. FIG. 21. 

 Pter'mopecteninvalidtiA, Hall, Pal. N. V., vol. v, pt. 1. Plates and Explanations : PI. 1, figf. is. Jan., 188 .'.. 



Shell small, rhomboidal; length and height nearly equal, moderately oblique; 

 margins regularly rounded, full behind; the greatest posterior extension i- 

 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. 



