712 BULLETIN UNITED STATE8 GEOLOGICAL SUEVEY. 



aud above the middle, but somewhat compressed toward the free margins, 

 especially in the posterior region ; front and posterior margins narrowly 

 and the basal broadly rounded, forming together a nearly true semi- 

 ellipse; cardinal margin broadly rounded and sloping gently downward 

 from the. beaks to the posterior margin; antero-dorsal margin slightly 

 concave just forward of the beaks, where the shell is also slightly con- 

 cave; umbonal portion of each valve prominent; beaks narrowed, dis- 

 tinctly defined, not much elevated, but pointing strongly forward and 

 incurved. Hinge and interior markings unknown. 



Length from front to rear 59 millimeters; height from base to beaks 

 38 millimeters ; greatest thickness, both valves together, 28 millimeters. 



In outward appearance, this species seems to occupy an intermediate 

 position between the usual short forms of CorMcula and that section of 

 the genus which was separated by the late Mr. Meek under the sub- 

 generic name of Leptesthes. It differs, however, from any species of that 

 section known to me in the narrowness and distinct definition of the 

 beaks ; the umbonal region being broad and the beaks depressed and 

 illy defined in all the published species of Leptesthes. 



With the exception of the differences named, and which seem to be 

 correlated subgeneric differences, the shell here described resembles in 

 general aspect some of the shorter varieties of Gorhicula {Leptesthes) 

 fracta Meek. For a more general comparison, however, it so nearly 

 resembles some species of Cardinia as to have suggested the specific 

 name which is here applied to it. 



Fosition and locality. — Laramie Group, Valley of Crow Creek, 15 miles 

 above its confluence with Platte Eiver, Northern Colorado, 



Corhiotda obesa (sp. nov.). 



Shell small or not above the average size for species of this genus, 

 inflated ; sides somewhat regularly convex, suboval, or subtrihedral in 

 marginal outline ; transverse length somewhat greater than the height; 

 basal margin almost regularly rounded, meeting both the posterior and 

 anterior margins by regular and nearly equal curves ; postero-dorsal 

 portion regularly rounded Irom the beaks to the posterior margin; antero- 

 cardiual margin straight, but the shell has a concave appearance in 

 front on account of the slight forward prominence of the beaks ; postero- 

 cardinal margin very little, if any, depressed below the adjacent portions 

 of the shell ; beaks small, pointed, not prominent, but directed a little 

 forward, and placed only a little in advance of the mid-length; lateral 

 teeth well developed, but slender, and apparently not crenulate, but the 

 condition of the examples in hand was not conclusive upon this point; 

 cardinal teeth well developed, but not robust; pallial sinus small. Sur- 

 face marked only by the usual lines of growth, and these being' mostly 

 very fine, the surface has a comparatively smooth, or sometimes even 

 a polished aspect in well-preserved examples. 



