LAMELLIBRANCHIATA. 181 



This species (lifters from L. longispinum in the wider ami somewhat less oljlique 

 1jo(I\ , with more reguhirh" romided posterior extremity ; the sinus in the wing 

 is less acute and farther below the cardinal line, also in the absence of the strong 

 posterior spine of that species. 



Formation and locality . In the Chemung group, near Elmira, N. Y. 



Leptodesma robustcm. 



I'LATE XXI, FIGS. 15, IK, -20; AM) FTw\TK LXXXIX. FIG. 8. 



Leptodesma mhitstuiii. Hall. Piil. N. V., vol. v, pt. 1. Plates and ICxplaiiatidiis : PI. 21. fiffs. l.'i, 16.20. 



Jan.. 1SS3. 



Shell large, sub-rhomboidal ; body obliquely ovate ; length much greater 

 than the height ; ante-byssal margin rounded ; straight below, or scarcely 

 impressed by the sinus, making a broad curve around the base to the post- 

 basal extremity and thence abruptly recurving toward the beak. 



Left valve gibbous above, regularly convex below the middle ; the greatest 

 convexity about one-third the length of the valve from the beak. Right 

 valve moderately convex in the middle. Hat below and scarcely gibbous on 

 the umbo. 



Hinge-line straight, about ecpial to the length of the shell. 



Beaks nearly anterior, obtuse, sub-erect, very prominent, rising above the 

 hinge-line. Umbonal region gibbous, prominent, ahruptly elevated along the 

 post-umbonal side for more than half the length of the shell, then merged in 

 the general convexity ; on the anterior side obscurely limited by a broad 

 undefined depression. Umbonal angle acute. 



Anterior extremity scarceh* auric ulate. Wing small, narrow-triangular, 

 extending about hall' the length of the valve, defined by the curving of the 

 striae; margin strongly concave, the deepest concavity just ])elow the hinge; 

 cardinal margin thickened and extended into a strong spine. In the right 

 valve the wing is larger and broader, and the sinus less deep ; the anterior 

 extremity corresponds with that of the left valve. 



Test moderately thick, marked by fine concentric striae which Ijecome 



