LAMELLIBRANCHIATA. - 481 



The specimen described is incomplete at the posterior extremity, and 

 had originally a length of about 42 mm. and a height of 5 mm. 



This species, in its narrow and extremely elongate form, resembles 0. cari- 

 nata, but differs in its ensiform, arcuate character, and the absence of promi- 

 nent carinae on the posterior slope. 



Formation and locality. In the shales of the Hamilton group, in Schoharie 

 county, N. Y. 



Orthonota rigida, n. sp. 



PLATE LXXX, FIG. 6. 



In part SanffuinolHes undatus, Hai,i,. Pal. N.*y., vol. v, i)t. 1. Plates and Explanations: PI. 80, 



ttgs. 5, 6. 1S83. 



Shell small, trapezoidal ; length more than twice the greatest height ; basal 

 margin straight in the posterior half, gradually curving upward toward the 

 anterior end. Posterior extremity nearly vertically truncate. Cardinal line 

 straight, extending for three-fourths the length of the shell. Anterior end 

 scarcely declining from the beaks and rounded below. 



Valves depressed-convex below, convex in the umbonal region. 



Beaks at about the anterior fourth, small, appressed, scarcely rising above 

 the hinge-line. Umbonal slope angular, extending to the post-inferior 

 extremity. Post-cardinal slope wide, depressed, marked along the centre by 

 a depression, which is defined in the anterior p.art and obscure toward the 

 posterior. 



Surface marked by fine concentric striae, and by regidar continuous con- 

 centric undulations, which are less conspicuous toward the basal and cardi- 

 n.al margins. 



The specimen described has a length of 28 mm. and a height of 12 mm. at 

 the posterior end. 



This species differs from O. undulata, in its shorter and broader form, the 

 umbonal slope is more distinctly angular, and is not limited by a groove below ; 

 the post-cardinal slope is not so strongly marked by the undulations and has no 

 distinct fold extending along the middle of its length. 



Formation and localiti/. In the Chemung group, near Elmira, N. Y. 



