282 PALEONTOLOGY OF NEW YORK. 



MODIOMORPHA CYMBULA. 



PLATE XXXVI, FIGS. 19, 20. 



MtMomorpha cynilmJa, Hall. Pi-elim. Notice Lamellibranchiata, 2, p. 75. 1870. 



•• " • " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 3(i, figs. 19, 20. 



1883. 



Shell of medium size, elongate, sub-elliptical ; length a little more than twice 

 the height ; basal margin regularly convex, acutely rounded at both extrem- 

 ities ; cardinal line regularly curving from the beaks to the posterior 

 extremity. Anterior end narrow, sub-nasute, without special limitation. 



Valves moderately and regularly convex, the greatest convexity being 

 about the middle of the length. 



Hinge-line oblique, curving ; length a little less than half the length of the 

 shell. 



Beaks sub-anterior, small, narrow, acute, closely appressed. Umbonal 

 region moderately convex, without any defined ridge, but the prominence 

 extends parallel to the cardinal line for more than two-thirds the length of 

 the shell. The anterior side of the umbonal region is flattened or depressed 

 in the cast, the depression extending downward nearly to the base, but not 

 affecting the margin. 



Test comparatively thick ; surface markings not preserved. The anterior 

 muscular impression is strong and large, with a small and deep retractor 

 scar. Pallial line parallel with the basal margin, abruptly recurving at the 

 posterior end and terminating in a large, shallow posterior adductor 

 impression. 



The specimen has a length of 60 mm., and a height of 28 mm. 



This form is more elliptical than any of the preceding species of the 

 Hamilton group, but is not so symmetrically elliptical as M. regularis and 

 M. linguiformis of the Helderberg limestone. It is distinguished from all the 

 species of the genus by its narrow and acute anterior and posterior extremities. 

 The species is of extremely rare occurrence, a single specimen only having 

 been observed in all the collections. 



