GEOLOGICAL SURVEY OF THE TERRITORIES. 495 



found wide sulcus from the beaks nearly or quite to the posterior basal 

 estreuiity, where the two are united. This character alone will distin- 

 guish that shell from the form here under consideration, whicli shows 

 nothing of the kind. Another well-marked difference is also observable 

 in the muscular im|)ressions; the anterior one in C. DurJceei being very 

 deep, while in the species here described it is so shallow as only to leave 

 rather faint marks of its outline on internal casts. 



Locality and position. — Cretaceous sandstones, including the coal at 

 Bear Eiver City, (Sulphur Creek,) Wyoming Territory. 



CORBICULA iEQUILATERALIS. 



Shell (as determined from an internal cast) subtrigonal, and nearly, 

 or quite, equilateral, rather convex; height about five-sixths the length ; 

 anterior and posterior extremities nearly equally, and rather narrowly, 

 rounded; ventral margin forming a nearly semielliptic curve, the most 

 prominent part being at the middle; beaks rather prominent, and very 

 nearly, if not quite, central; umboual slopes not prominently rounded; 

 dorsal outline declining subequally from the beaks in front and rear, the 

 posterior slope being convex in outline, and the anterior concave ; mus- 

 cular impressions shallow. (Surface and hinge unknown.) 



Length, 1.72 inches; height, 1.45 inches; convexity, about 0.92 inch. 



One specimen of this shell shows impressions in the matrix of elongated 

 lateral teeth, like those of Corbicula in form ; but the arenaceous ma- 

 terial is too coarse to have defined the striations of these teeth, if any 

 existed. Until the cardinal teeth can be seen, its relations to that genus 

 cannot be positively determined, though I have little doubt that it 

 belongs to that group. It will be readily distinguished from the last by 

 its less elongated and equilateral form. These characters will also 

 equally distinguish it from (7. Durlceei, from the Bear Eiver estuary 

 beds. 



Locality and position. — Same as last. 



Cyrena Carletoni, Meek. 



Shell small, thin, subcircular, or with length a little greater than the 

 height ; moderately convex ; anterior and posterior margins rounding 

 from above regularly into the rounded basal outline, or with the i)osterior 

 sometimes slightly straightened, both rounding more abruptly to the 

 hinge above ; beaks rather depressed, small, abruptly pointed, incurved, 

 nearly contiguous, and placed slightly in advance of the middle ; hinge 

 line sloping very gradually from the beaks. Surface marked with mod- 

 erately distinct concentric lines and furrows. 



Length of a medium-sized specimen, 0.55 inch ; height of same, 0.49 

 inch ; convexity, 0,32 inch. 



This shell is so very thin, and so nearly resembles a rather large 

 Splioirium in form and surface characters, that I should certainly have 

 referred it to tliat genus, had not a lucky blow separated the hinge of 

 a right valve from the matrix in such a manner as to expose the teeth 

 quite satisfactorily. This shows its hinge to have the characters of a 

 tY\xe> Cyrena. -For so thin a shell it has quite a stout hinge. Its car- 

 dinal teeth are rather diverging, the posterior two being well developed, 

 and each a little furrowed along the middle, while the anterior one (in 

 this rtght valve) is much smaller and conical in form. The lateral 

 teeth are of moderate size, and certainly smooth, the posterior being 

 remote from the cardinal teeth, and the linear anterior extending back 



