CLINTON, LOWER UELDERBERG, CHEJIUNG, AND WAVERLY GROUPS. 449 



Right valve unknown. Left valve moderately convex; greatest convexity at 

 the unibonal retrions. 



Hinge line essentially straight; length a little less than that of 

 the body. 



Beak situated at about the anterior fourth of the shell, small, (\ 

 extending beyond the hinge line. A shallow broad sinus extends 

 from the beak to the basal margin a little anterior to the middle. *ps t- 



Anterior extremity acuminate, margin rounded. 



"Wing triangular, joining the body at the posterior extremity ; margin for a short 

 distance straight, then abruptly curving forward, and just before reaching the cai- 

 dinal line curving upward. Wing convex ; flattened immediately below the cardinal 

 line; separated fi'om the body .by a narrow but conspicuous depression. 



Surface ornamented by faint radiating strite, which arc obsolete, except on i)ei-- 

 fectly i)reserved specimens, and also by fine concentric striie, which frequently become 

 fasciculate on the anterior portion of the shell. 



This species may be distinguished from any other at present known by the slight 

 obliqueness of the body of the shell, the basal margin being nearly parallel with the 

 hinge line; the margin is decidedly convex; in nearly all other species it is straight 

 or concave ; the wing extends beyond the body of the shell. The outline is very 

 similar to that of some forms of the genus Cypricardinia. 



Formation and locality. Chemung group, hill north of Warren, Warren county, 

 Pennsylvania. 



MODIOMORPHA RIGIDULA Simpson, n. sp., Fig. 16. 



Shell of medium size or smaller, subquadrangular in outline ; height a little more 

 than three-fifths the length of the shell ; basal margin regularly and gently curving 

 from the anterior to the postbasal extremity ; posterior margin gently ..i.y ^r-"^ - 

 curved, slightly o1)lique, sometimes neaily at right angles to the basal 

 margin; cardinal line essentially straight ; anterior rounded al)rui)tly, '/ 



extended, without limitation by a sinus. ""' -' 



Beaks a little more than one-fourth the length of the shell from the anterior enil; 

 umbonal ridge prominent, extending from the beaks to the postbasal extremity. 

 Valves convex towards the basal margin, becoming gibbous above the middle and in 

 the umbonal region ; po.sterior slope convex near the beaks, becoming fiattened as it 

 approaches the posterior margin. 



Surface marked by concentric stritc which frequently become obsolete on poriion.s 

 of the shell. 



A. p. s. — VOL. xvr. yK. 



