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PALAEONTOLOGY OF NEW YORK. 
PTYCHODESMA, Hall and Whitfield. 1872. 
Ptychodesma Knappianum. 
PLATE LI, PIGS. 22-27. 
Ptychodesma Knappianum, Halt, and Whitfield. Twenty-fourth Ann. Rep. N. Y. State Mus. Nat. Hist., 
p. 192. 1872. 
“ “ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PL 
51, figs. 22-27. 18S3. 
Shell obliquely sub-ovate ; length usually more than one-third greater than the 
height; basal margin oblique, often nearly straight in the middle of its length. 
Posterior end broadly rounded. Cardinal line straight, oblique, having a 
length posterior to the beaks of about one-half the length of the shell. 
Anterior end very short, curving downward into the basal margin without 
limitation. 
Valves convex in the lower and posterior portions, gibbous in the middle 
and above. 
Beaks sub-anterior, small, distant, closely incurved, rising but little above 
the hinge-line. Umbonal slope presenting an undefined ridge, which merges 
into the general convexity of the shell about the middle of its length. 
Test thick, marked by fine concentric strias, with distant imbricating 
lamellae. The anterior and basal margins are thickened and present the 
edges of numerous lamellae of growth. 
Ligamental area deeply excavated, marked by numerous longitudinal 
striae, which are abruptly arched just beneath the beak. Hinge with two 
or more cardinal teeth. Anterior muscular impression strong. 
Three specimens measure respectively 43, 38 and 35 mm. in length, and 
28, 28 and 27 mm. in height. 
This shell bears some resemblance externally to several forms of Modiomorpha 
and Nyassa, but the deeply grooved ligamental area is a distinctive feature. 
Formation and localities. In the clierty layers of the age of the Hamilton 
group, from the Falls of the Ohio, and Dublin, Ohio. A form of this genus 
occurs in the Hamilton group, in the State of New York, at several localities; 
