LAMELLIBRANCHIATA. . 283 



Formation and locality. In the arenaceous shales of the Hamilton group, at 

 East Worcester, Otsego county, N. Y., associated with characteristic Hamilton 

 fossils. 



MODIOMORPHA SUBALATA. 



PLATE XXXV, KIGS. 6, 7 ; AND PLATE XXXLX, FIGS. 1-U, 16. 



Cypricardites gubalata, Conrad. Geol. Sui-v. N. Y., Ann. Rep., p. 83. 1841. ^ 

 Modiotnorpha subcUata (Cosbad), Hall. Prelim. Notice Lamellibranchiata, 2, p. 77. 1870. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 39, figs. 



1-14, 16. 1S83. 



Shell of medium size or smaller, sub-quadrangular, or sub-ovate; sub-alate 

 posteriorly, with the extremity obliquely truncate ; length more than one- 

 third greater than the height ; basal margin straight or slightly concave on 

 the anterior half, and thence regularly curving to the post-basal extremity ; 

 posterior margin abruptly recurved below, and continuing with a gentle 

 curvature, or in a nearly straight line, to the cardinal margin ; cardinal mar- 

 gin oblique, nearly straight; post-cardinal area sub-alate. Anterior end 

 short, abruptly rounded, often a little concave below the beaks, and limited 

 by a more or less defined sinus, which extends from the beak to the basal 

 margin, usually at a point less than one-third the length of the shell from 

 the anterior end. 



Valves moderately convex below and in the posterior portion, more convex 

 in the middle. 



Hinge-line oblique, about half the length of the shell. 



Beaks sub-anterior, incurved and directed forward, somewhat compressed. 

 Umbo prominent, flattened anteriorly ; the post-umbonal slope angular, gib- 

 bous above and gradually declining to the post-basal extremity, which is 

 sometimes obtusely angular. 



Test thin, marked by fine concentric striae, which are sometimes fascicu- 

 late, forming distinct elevations upon the posterior slope, and often distinct 

 angular striae upon the antero-basal portion of the shell, frequently becom- 

 ing nearly obsolete on the umbonal region. 



Anterior muscular impression circular, of moderate strength ; other interior 

 characters undetermined. 



