MOLLUSCA. 645 



margin regularly rounded from the cardinal to the basal margin ; 

 ventral margin gently convex, with a slight sinuosity just in front 

 of the posterior extremity; postero-basal extremity angular; 

 posterior margin short, truncate, slightly oblique; postero-car- 

 dinal extremity obtusely angular; postero^cardinal margin 

 nearly straight. Right valve with a low, angular, slightly 

 sigmoidal umbonal ridge; the most prominent portion of the 

 valve belo>w the beak, the surface curving somewhat abruptly 

 to the antero-cardinal margin and more gently towards the 

 umbonal ridge, just in front of which, in the lower half of 

 the shell, is a shallow and inconspicuous sinus; post-umbonal 

 slope rather gentle, concave. Surface of the valve marked by 

 fine, concentric costse, which become more pronounced towards 

 the ventral margin and upon the post-umbonal slope. Left valve 

 not known. 



Remarks. — This species is described from a nearly perfect 

 right valve, from the Marshalltown clay-marl near Swedesboro. 

 The shell substance is preserved and all the external characters 

 are clearly visible. The shell resembles C. subgibbosa Con., in 

 general form, but has the beaks pointing forward instead of pos- 

 teriorly, as in that species. It also resembles the original illus- 

 tration of C. subcompressa Gabb, but it has the beak further 

 forward and the posterior margin less oblique. 



A single internal cast of a Corbula from the Cliffwood clay has 

 been referred to this species ; the beak is somewhat more obtuse^ 

 the umbonal ridge less angular, and the posterior margin some- 

 what narrower than in the specimen preserving the shell, but 

 these are all differences which might be expected in a cast of the 

 interior of the shell. Another somewhat distorted internal cast 

 from the Woodbury clay near Haddonfield, is also placed in this 

 same species. Both of these internal casts retain traces of the 

 concentric surface markings, which agree closely with those of 

 the shell itself. 



Formation and locality. — -tliffwood clay, near Matawan 

 (189); Merchantville clay-marl, near Matawan (loi); Wood- 

 bury clay, near Haddonfield (165); Marshalltown clay-marl, 

 near SwedesborO' (177). 



Geographic distribution. — New Jersey. 



