LAMELLIBRANCHIATA. 355 



extremity. Anterior muscular impression deep and strong. Pallial line sim- 

 ple, often strongly marked in the anterior part of the shell. Posterior mus- 

 cular impression situated on the cardinal slope, not strongly marked. Liga- 

 ment narrow, external. 



Five specimens measure respectively 21, 28, 45, 47 and 52 mm. in length, 

 and 11, 16, 22.5, 24 and 24.5 mm. in height. 



This species is distinguished from N. recta by its arcuate, sub-elliptical form, 

 sinuate basal margin, and larger size. 



Formation and localities. In the arenaceous shales of the Hamilton group, in 

 Schoharie, Otsego, Madison and Onondaga counties, and abundant in some cal- 

 careous layers of the group at Pratt's falls, in Onondaga county, N. Y. 



Nyassa subalata. 



PLATE LUI, FIGS. 21-26. 



Nyassa subalata. Hall. Prelim. Notice Lamellibranchiata, 2, p. 29. 1870. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations; PI. 53, fig.s. 21-26. 1883. 



Shell of medium size or larger; body broadly sub-ovate, and including the 

 alation, rhoinboidal in outline; length one-third greater than the height; 

 basal margin regularly rounded and sometimes nearly straight in the middle, 

 abruptly recurving at the post-inferior extremity; posterior end oblique, 

 broadly truncate, with the margin slightly convex. Cardinal line oblique, 

 usually alate posteriorly. Anterior end short, narrowed and regularly rounded 

 below the beak. 



Valves moderately convex below, becoming gibbous above and in the 

 umbonal region. 



Beaks sub-anterior, small, appressed, rising but little above the hinge-line. 

 Umbonal ridge rounded, undefined, merging into the general convexity of 

 the shell before reaching the post-inferior extremity. 



Test thick anteriorly, marked by fine, irregular, concentric strioa which 

 are often raised into strong fascicles along the basal and posterior margins. 

 A single young individual shows distinct, continuous, distant radii on the 



