LAMELLIBRANCHIATA. 181 



This species differs from L.kmgispinum in the wider and somewhat less oblique 

 body, with more regularly rounded 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 -roup, near Klniira. X. Y. 



LEPTODESM \ ROBUSTDM. 



PLATE XXI, FIGS r>. 16, 20; vm. II. ATI. LXXXIX, FIG. 8 



Leptodesma vobustum, Hall. Pal. N. V., vol. v. pt. 1. Plates and Explanations: PI. 21. figs. 15, 16. 20 



Jan., 1883. 



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 equal to the length of the shell. 



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

 hinge-line. Umbonal region gibbous, prominent, abruptly 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 scarcely auriculate. Wing small, narrow-triangular, 

 extending about half the length of the valve, defined by the curving of the 

 striae; margin strongly concave, the deepest concavity just below 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 dee]) : the anterior 

 extremity corresponds with that of the left valve. 



Test moderately thick, marked by fine concentric stria' which become 



