LAMELLIBRANCHIATA. 405 



above the hinge-line. Umbonal slope distinctly angular, extending to the 

 post-inferior extremity. Post-cardinal slope flat or convex, with an obscure 

 fold along the middle, which may be obsolete in the cast. Cincture not 

 conspicuous, consisting of a slight depression or constriction extending from 

 the beaks oblifiuely backward to about the middle of the shell. 



Surface marked by fine concentric stria?, which become fascicled, produc- 

 ing low undulations, which are usually more conspicuous on the anterior and 

 posterior portions of the shell ; also obscurely marked, in the cast, by fine 

 ra<liating striae posterior to the cincture. Interior unknown. 



Three specimens measure respectively 50, 44 and 37 mm. in length, and 

 22, 20 and 16 mm. in height. 



This species closely resembles S. rigidus, but the cincture is more oblique, 

 extending further backward on the basal margin ; the post-cardinal slope is 

 narrower and less distinctly marked by a fold along the middle. 



Formation and locality. In the Waverly sandstone, at Newark, 0. 



Sphenotus signatcs, n. sp. 



PLATE XCIV, FIG. 3. 



Shell large, very elongate-elliptical ; length nearly three times the height ; 

 basal margin gently curving. Posterior extremity doubly truncate, vertical 

 below and oblique above. Cardinal line long, very gently arcuate. Ante- 

 rior end produced, declining from the beaks and narrowly rounded. 



Valves moderately convex along the basal and posterior portions, becom- 

 ing gibbous in the middle. 



Beaks sub-anterior, small, but little elevated above the hinge. Umbonal 

 ridge rounded, scarcely angular, extending to the post-inferior extremity. 

 A sub-angular fold extends along the middle of the post-cardinal slope. 



Surface marked by fine concentric striae, which are distinctly undulated 

 in passing over the umbonal and cardinal ridges. 



The specimen described has a length of 55 mm. and a height of 20 mm. 

 52 



