LAMELLIBRANCHIATA. 287 



MODIOMORPHA SDBANGULATA, n. Sp. 

 PLATE XXXV, FIGS. 10, 11. 



Shell of medium size, elongate sub-ovate, wider behind ; length more than 

 twice the height ; basal margin slightly arcuate, abruptly rounding at the 

 extremities; posterior margin acutely rounded below and obliquely sub- 

 truncate to the cardinal line ; cardinal margin nearly straight. Anterior 

 end rapidly declining from the beak and abruptly rounded below. 



Valves moderately convex in the posterior part, and gibbous and sub- 

 angular in the umbonal region. There is a broad depression or sinus which 

 extends from just posterior to the beak to the basal margin near the middle 

 of the length of the shell. 



Hinge-line slightly oblique, extending about half the length of the shell. 



Beaks almost anterior, prominent, small, angular, incurved and elevated 

 above the hinge-line. Umbonal region gibbous, with a prominent ridge 

 extending from the beaks to the post-basal extremity, angular in its upper 

 portion, becoming more subdued toward the posterior end. 



Test thick, marked by fine concentric striaa, which are sometimes fascicu- 

 late, making stronger ridges on the surface. Anterior end marked by a 

 strong, muscular impression just below the beak, close to the anterior margin. 



A large individual of this species has a length of 55 mm., and a height of 

 26 mm. A smaller specimen has a length of 39 mm., and a height of 19 mm. 



This species bears some resemblance to M. subalata, but differs in its much 

 stronger and more angular umbonal ridge, which reaches to the post-basal 

 extremity, the basal margin is more arcuate, the cardinal and basal margins 

 are more nearly parallel, and the anterior end is shorter. 



Formation and locality. In the Chemung group, Mansfield, Tioga county. Pa. 



MODIOMOEPHA RIGIDA. 

 PLATE XLI, FIGS. 11-16. 



Modiomorpha rigida. Hall. Pal. N. Y., voL v, pt. 1. Plates and Explanations : PI. 41, figs. 10, 11, 14-16 



(figs. 10, 11 in error). 1883. 



Shell of medium size or smaller, sub-rhomboid-ovate; length nearly twice 

 the height; basal margin regularly and gently curving from the anterior 



