352 PAI.JEONTOLOGY OF NEW YORK. 



PTYCHODESMA, Hall and Whitfield. 1872. 

 Ptychodesma Knappianum. 



IM.ATE 1,1, FIUS. 22-27. 



Ptydiode»ua KiiaypiaHum, Halt, and Whitfikld. Twenty-foui-th Ann. Rep. N. Y. State Mns. Nat. Hist., 



I>. 192. 1872. 

 " " " " Pal. N. y., vol. V, pt. 1. Plates and Explanations: PI. 



51, figs. 22-27. 1883. 



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 striae, 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 cherty 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; 



