Lamellibranchiata. 383 



known. It differs sufficiently from the other forms here described to 

 be easily recognized. 



Formation and locality. In a white sandstone of the middle por- 

 tion of the Chemung group near Angelica, Alleghany county, N. Y. 



Leptodesma rude. 



Leptodesma rude, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 25, fig. 12. Jan., 1883. 



Shell large, sub-rhomboidal ; body broadly ovate below, rapidly 

 attenuating above, oblique to the hinge-line at an angle of about 

 50°; length less than one-third greater than the height ; ante-byssal 

 margin curving into the broad sinus ; ventral and posterior margins 

 broadly rounded. Left valve convex below, becoming narrow and 

 gibbous above the middle. Right valve less convex. Hinge-line 

 straight, less than the length of the -valve. Beak sub-anterior, 

 directed forward, acute, little elevated above the cardinal line. 

 Umbonal region narrow and gibbous, subtending an acute angle. 

 Anterior end large, limited by a vertical byssal depression ; extremity 

 acute. Wing of moderate size, joining the body at the posterior 

 extremity ; margin concave ; termination acute. 



Test thick, marked by concentric striae, which have been fascicu- 

 late, leaving strong undulations upon the cast of the interior. 

 These surface characters are subdued in the right valve. 



A left valve has a length of 51 mm,, height 37 mm., hinge-line 

 about 42 mm. 



Specimens of this species usually present a very rough and irregular 

 aspect from the character of the shell, with its strongly lamellose 

 structure and the coarse matrix in which it is usually found. In a 

 crushed specimen retaining both valves, the right valve is almost 

 equally convex with the left ; the surface is less lamellose and presents 

 only gentle undulations. 



Formation and. locality. In a coarse sandstone of the upper part of 

 the Chemung group, on the road from Olean, N". Y., to Smethport, Pa. 



Leptodesma Bitok. 



Leptodesma Biton, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell large, sub-rhomboidal ; body elongate-ovate, making an angle 

 of about 45° with the hinge-line ; height nearly two- thirds of the 

 length; ante-byssal margin oblique, slightly curving into a long, 



