LAMELLIBRANCHIATA. 353 



but the material at hand is too obscure to warrant its separation as a distinct 

 species. 



Ptychodesma minor. 



PLATE XLI, FIG. 27. 

 PlyclwXenmai vUiwr, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations; PI. 41, fig-. 27. 18S3. 



Shell small, sub-ovate ; length about one-third greater than the height; basal 

 margin regularly rounded. Posterior extremity somewhat abruptly rounded, 

 and extended in the middle. Cardinal line short, oblique. Anterior end 

 short, regularly rounded below the beaks. 



Valves regularly convex in the basal and posterior portions, gibbous in the 

 middle and in the umbonal region. 



Beaks anterior, jjrominent, incurved. An undefined sub-angular umbonal 

 ridge extends to the post-inferior extremity of the shell. 



Surface marked by fine concentric striae. Interior unknown. 



The specimen figured has a length of 16.5 mm., and a height of 11 mm. 



This species is distinguished by the slight obliquity of the body of the shell 

 to the hinge-line, the extension of the shell in the middle of the posterior mar- 

 gin, the forward direction of the beaks, and the strong umbonal ridge. 



Formation and locaWy. In the shales of the Chemung group, near Elmira, N. Y. 



Ptychodesma nanum, n. sp. 



PLATE XCIII, FIGS. 17, 18. 



Shell small, broad-ovate; length about one-fourth greater than the height; 

 basal margin broadly rounded posteriorly, with a sinus toward the anterior 

 end. Posterior extremity regularly and broadly rounded, recurving to the 

 cardinal line. Cardinal line short, oblique. Anterior end short and abruptly 

 rounded below the beak. 



Valves moderately convex in the posterior and lower part, becoming gib- 

 bous in the umbonal region. 



Beaks anterior, directed upward, rising but little above the hinge-line. 

 Umbonal slope rounded. 



