366 Thirty-fifth Report on the State Museum. 



tricose above the lower third. Right valve unknown. Hinge-line 

 straight ; length less than the length of the shell. Beak at about 

 the anterior third of the cardinal line, directed slightly forward, 

 prominent and arching over the hinge. Umbonal region narrow 

 and ventricose, denned anteriorly by a broad, shallow depression, 

 which is nearly vertical to the hinge-line, and posteriorly by the 

 abrupt depression of the body. Anterior end acute, triangular. 

 Wing rather large, triangular, convex, joining the body of the shell 

 near the posterior extremity ; margin broadly concave ; extremity 

 produced, acute. 



Test thin, marked by fine concentric lines of growth which are 

 irregularly crowded into small fascicles, producing a slightly undu- 

 lated appearance. The striae are strong and regular over the wing. 



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. 

 rohustum, but the body 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. robus- 

 turn. It is very closely allied to L. umbonatum var. depressum, but 

 differs in its smaller mucronate anterior end ; the body is more con- 

 vex and narrower below, and the extremity of the wing more produced. 



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

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



Leptodesma umbonatum, var. depressum. 



Leptodesma umbonatum, var. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Ex- 

 planations : 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, below 

 the acute extremity, regularly curving into the ventral margin ; 

 posterior extremity produced, with the margin gently recurving into 

 the wing. Left valve convex below, 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 by 

 a broad, undefined depression ; extremity produced, acute. Wing 

 lar*ge, triangular, not distinctly defined, joining the body near the 

 posterior end ; extremity produced, acute, and probably extended 

 into a short spine. 



