LAMELLIBRANCHIA TA. 
353 
but the material at hand is too obscure to warrant its separation as a distinct 
species. 
Ptychodesma minor. 
PLATE XLI, FIG. 27. 
Ptychodesma? minor, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PL 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, prominent, 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 locaJity. In the shales of the Chemung group, near Elmira, N. Y. 
Ptychodesma nanum, n. sp. 
PLATE A cm, FIGS. 17, IS. 
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. 
