LAMELLIBRANCHIATA. 199 



Test thin, marked by fine concentric lines of growtli whicli are irregularly 

 crowded into small fascicles, producing a slightly undulated appearance. 

 The stria? are strong and regular over the wing. 



Interior unknown. 



The left valve described has a length of 41 mm., height 27 mm., hinge-line 

 about 38 mm. 



This species bears some general resemblance to L. potens and L. robustum, but 

 the l)ody is narrower, less oblique and much more gibbous, the beak more 

 elevated, and the posterior extremity of the body less expanded. The wing 

 is also much larger than in L. robustum. It is very closely allied to L. umbonatum 

 var. depressu?n, PL xxii, fig. 14., but differs in its smaller mucronate anterior 

 end ; the body is more convex and narrower below, and the extremity of the 

 wing more produced. 



Formation and locality. In a calcareous band of the ujjper part of the 

 Chemung group. Twenty-mile creek, Chautauqua county, N. Y. 



Leptodesma umbonatum, var. depressum. 



PLATE XXn, FIG. 14 ; AND PLATE XC, FIG. 10. 



Leptodesma wnhonatum, var. depressum, Hall. Pal. N. Y., vol. v. [it. 1. Plates and Exjilanaticus: PI. 22. 



fig. 14. Jan., 1883. 



Shell of medium size, rhomboidal ; body very oblique-ovate ; length once and 

 a half greater than the height ; ante-byssal margin, Ijelow the acute extrem- 

 ity, regularly curving into the ventral margin ; posterior extremity produced, 

 with the margin gently recurving into the wing. 



Left vafve convex l)elow, gibbous above. Right valve smaller, a little less 

 convex, abruptly gibbous on the umbo. 



Hinge-line straight, somewhat less than the greatest length of the shell. 



Beaks sub-anterior, obtuse, directed slightly forward, very prominent, 

 arching over the hinge-line. 



Anterior end short, large, limited liy a broad, undefined depression ; 

 extremity produced, acute. Wing large, triangular, not distinctly defined. 



