LAMELLIBRANCHIATA. 357 



Surface marked by fine, closely arranged, concentric striae, which become 

 crowded and fascicled at irregular distances. 



.Anterior muscular impression strongly marked, situated close within the 

 anterior margin of the shell. The posterior extension of the cardinal line is 

 marked by two distinct lateral teeth. 



Four specimens measure respectively 20, 21, 24 and 28 mm. in length, and 

 10, 11, 12 and 15 mm. in height. 



This species is distinguished from N. arguta in its smaller size ; by its 

 straight hinge-line, more prominent beaks, the direct umbonal ridge and the 

 absence* of distinct constriction in the basal margin. 



Formation and localities. In the softer shales and also the arenaceous beds of 

 the Hamilton group, near Fultonham and Summit, Schoharie county, N. Y. 



Nyassa elliptica. 



TLATE XXXIV, FIG. 8. 



Nyassa elliptica. Hall. Prelim. Notice Lamellibranchiata, 2, p. 30. 1S70. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. 34, fig. 8. 1883. 



Shell of medium size, broadly elliptical in outline ; length and height as 7 to 

 4; basal margin regularly curving; posterior extremity regularly rounded. 

 Cardinal line gently arcuate from the beaks to the posterior extremity. 

 Anterior end short, regularly rounded below the beak. 



Posterior and basal portions of the shell gently convex, becoming more 

 convex above and morterately gibbous in the umbonal region. 



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

 Umbonal slope marked by an undefined rounded ridge which extends toward 

 the post-inferior extremity, but merges into the general convexity a little 

 beyond the middle ; sometimes a slight depression below the umbonal ridge, 

 extending half way to the post-inferior extremity. 



Test of moderate thickness, marked by fine concentric striae of growth. 

 The specimen is a cast of the interior, preserving a small portion of the shell. 



Anterior muscular impression strong, situated just within the border of 

 46 



