356 PALEONTOLOGY OF NEW YORK. 



posterior slope reaching to the posterior margin. This feature is also 

 obscurely shown on several of the older individuals (erroneously represented 

 in fig. 21). 



Cardinal teeth not determined. One or two lateral teeth are present on 

 the cardinal alation, extending nearly to the posterior extremity. Anterior 

 muscular impression strong. 



Five characteristic specimens measure respectively 23, 36, 38, 39 and 41 

 mm. in length, and 16, 24, 25, 27 and 28 mm. in height. 



This species is readily distinguished from the known species of the genus by 

 its short, broad form and alate post-cardinal extension. 



Formation and localities. In the shales of the Hamilton group, at Fultonham 

 and Summit, Schoharie county, N. Y. 



Nyassa recta. 



PLATE LIU, FIGS. 1-6. 



Nyassa recta. Hall. Pi-elim. Notice Lamellibranchiata, 2, p. 29. 1870. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 53, figs. 1-8. 1883. 



Shell small or of medium size, ovate sub-cuneate ; length nearly twice the 

 height ; basal and cardinal margins sub-parallel ; basal margin gently curv- 

 ing or often nearly straight, abruptly recurved at the post-inferior angle and 

 more gently curving at the anterior end. Posterior extremity obliquely 

 truncate. Cardinal line straight or slightly arching, and declining toward 

 the posterior end. Anterior end short, narrowed and regularly rounded 

 below the beaks. 



Valves regularly convex in the lower part, gibbous above and in the 

 umbonal region. 



Beaks sub-anterior, prominent, rising above the hinge-line. Umbonal 

 ridge direct to the post-inferior extremity, with usually a slight flattening or 

 depression in the shell below the ridge, which extends from the umbo to the 

 base, scarcely constricting the margin. 



