LAMELLIBRANCHIATA. 221 



The specimen described is 60 min. in length, with a height of 29 mm., 

 and hinge-line 61 mm. 



This snecies bears considerable resemblance to L. extmuatum in the characters 



J. 



of the right valve ; bnt it i-s proportionally narrower, the posterior exti'emity 

 more abruptly recurved and the hinge-line less extended behind. A single 

 right valve is the only specimen at present known. It differs sufficiently from 

 the other forms here described to be easily recognized. 



Formation and locality. In a white sandstone of the middle portion of the 

 Chemung group near Angelica, Alleghany county, N. Y. 



Leptodesma rude. 



PLATE XXV, FIG. \i ; and PLATE XCI, FIG. 3. 

 Leptodesma rude, Hxhh. Pal. N. Y., vol. v, jit. 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 strise, which have been fasciculate, 

 leaving strong undulations upon the cast of the interior. These surface 

 characters are subdued in the right valve. 



Interior unknown. 



