Lamellibkanchiata. 367 



Test marked by fine striae of growth which are irregularly fascic- 

 ulate, producing a slight undulation of the surface. The striae are 

 lamellose on the anterior side and regular on the wing. 



The specimen described has a greatest length of about 40 mm., 

 height 25 mm., and hinge-line 35 mm. 



This form differs from L. umbonatum in the lesser convexity and 

 greater width of the lower part of the body ; less abrupt depression 

 on the anterior side from the umbo ; the posterior slope less ele- 

 vated and defined below the middle, and a much larger ante-byssal 

 portion of the shell. 



Formation and locality. In the upper part of the Chemung group, 

 Napoli Centre, Cattaraugus county, N. Y. 



Leptodesma navifokme. 



Leptodesma naviforme, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 22, fig. 15 ; pi. 23, fig. 1. Jan., 1883. 



Shell below the medium size, rhomboidal ; body very oblique, short- 

 ovate ; length more than one-third greater than the height ; ante- 

 rior extremity subtruncate; margin curving to the broadly rounded 

 base ; posterior margin slightly produced, gently curving forward, 

 nearly vertical. Left valve convex below, very gibbous above the 

 middle. Eight valve depressed-convex below, gibbous above. Hinge- 

 line straight, more than one-third greater than the height of the 

 shell. Beaks obtuse, situated at the anterior fourth of the hinge-line, 

 prominent, directed slightly forward. Umbonal region gibbous, slop- 

 ing abruptly into the wing. Anterior end short, acute. Wing 

 large, not distinctly limited, extending almost to the posterior ex- 

 tremity ; margin scarcely concave below ; extremity produced, acute. 

 Test marked by fine concentric striae, which are crowded into 

 fascicles at nearly equal intervals, rounded upon the upper part of the 

 body, and sub-angular on the lower part. The striae are crowded and 

 lamellose on the anterior ; on the posterior slope they make a short 

 abrupt curve, passing over the wing with a gently forward direction 

 and curving backward just below the hinge-line. In the weathered 

 surface of the right valve the concentric undulations are stronger, 

 the post-cardinal slope is marked by strong interrupted radii which 

 appear to belong to the intimate structure of the shell. The hinge 

 is marked by a single narrow groove. 



A specimen of the left valve has a length of 22 mm., height 14 

 mm., and hinge-line about 21 mm. A right valve occurring in the 

 same association has a length, from beak to base, of 26 mm., height 

 18 mm., and hinge-line about 25 mm. 



