LAMELLIBRANCHIATA. 291 



Valves moderately convex in the posterior part, gibbous in the middle and 

 umbonal region. 



Hinge-line short, oblique, about one-third the length of the shell. 



Beaks anterior, prominent, small, distinct. Umbonal slope rounded, flat- 

 tened on the anterior side. 



Test thin, marked by fine concentric striae. Interior unknown. 



The specimen described has a length of 24 mm., and a height of 15 mm. 



This form is distinguished from M. subalata, var. Chemungensis, by the absence 

 of defined umbonal ridge, and in the regularly rounded posterior extremity. 



Formation and locality. In the Chemung group. Buck's quarry, near Elmira, 

 N. Y., associated with Orthis impressa, Atrypa hystrix, and Produdella Boydi. 



MoDioMORPHA Tioga, n. sp. 



PLATE XL, FIG. 18. 



Modiomorphaf amygdalbia fWiifCHBLL, sp ), Hah. Prelim. Notice Lamellibranchiata, 2, p. 78. 1870. 

 In part " " " " Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 



PL 40, figs. 15-19. 1883. 

 Not SanguijiolUes amygdedimts, Winchell. Proc. Acad. Nat. Sci. Phila , p. 13. 1863. 



Shell of medium size, sub-ovate, wider behind, sub-truncate at both extrem- 

 ities ; length one-half greater than the height ; basal margin slightly sinuate 

 in the middle, curving toward the extremities, and abruptly rounded into 

 the anterior and posterior margins. Posterior margin sub-truncate in the 

 upper portion. Anterior end declining abruptly from the beak and narrowly 

 rounded in front, limited by an obscure depression extending from the beak 

 to the basal margin. 



Valves depressed-convex posteriorly and in the lower part, moderately 

 convex in the umbonal region. 



Hinge-line imperfect in the specimen described. 



Beaks situated a little less than about one-fourth the length of the shell 

 from the anterior end, small, closely appressed, rising very little above 

 the hinge. 



Test thin, marked by fine concentric stria3, which, in some parts, have 



