18(1 PAL.KOSTOLOGY OF NEW YORK. 



valves are the same in form; the sinus in the wing of the right valve is not 

 so deep as in the left valve but much deeper than in the right valve of that 

 species. 



The original specimen is illustrated in figure 14 of plate \\i. and figures 17. 

 18, 19 arc right valves occurring in the same beds. 



This form is associated with Avicula spinigera of Conrad, which is a smaller 

 shell, less oblique, narrower, and with a more angular anterior extremity. 



Formation and localities. In the Chemung group, at Painted Post. Steuben 

 county, between Corning and Elmira, and at Chemung Narrows. N. Y. 



Leptodesm \ Shumardi, n. sp, 



PLATE I. XXXIX. PIGS 5, B, 



Shell of medium size, sub-rhomboidal ; body narrow, very obliquely-ovate; 

 length somewhat less than twice the height; margin very broadly curving 

 from the anterior extremity to the base; basal margin rounded and abruptly 

 recurved at the post-basal extremity. 



Left valve quite convex; the greatest convexity about one-third the length 

 of the valve below the apex. Right valve unknown. 



Hinge-line straight ; length about equal to the height of the shell. 



Beak sub-anterior, obtuse, prominent and arching over the hinge-line. 

 Umbonal region gibbous, subtending an acute angle. 



Anterior extremity short, acute, straight above, oblique below. Wing 

 small, narrow-triangular, extending to a point below the middle of the length 

 of the shell: margin concave: extremity acute, probably spiniform. 



Test thin, marked by lamellose, often fasciculate stria-, which are more 

 crowded on the anterior side and byssal depression, and more regular and 

 distinct on the wing. 



I nterior unknown. 



Three specimens of left valves have respectively the following dimensions : 

 length 12, 10 and 36 nun., height 24, 27 and 24 nun., and hinge-line about 

 26, 2 1 and "J 1 mm. 



