514 GEOLOGICAL SURVEY OF THE TERRITORIES. 



valve about one-fourth less convex than the other, with posterior umbo- 

 nai slope distinctly angular from the beak to the posterior basal extrem- 

 ity; right valve with posterior umbonal slope less strongly augulated; 

 surface of both valves ornamented with concentric ridges, geueraliy small 

 and regular on the umbonal region, but often swelling out into a few very 

 prominent, angular folds with rounded depressions, marked by distinct 

 lines of growth, and some small ridges between, on the lower half of the 

 valves ; all the ridges and folds generally becoming obsolete behind the 

 angular, posterior, umbonal slopes, but continued forward to the front. 



Length of a well-developed specimen, 0.76 inch; height to top of 

 umbo of left valve, 0.50 ; height to top of same of right valve, 0.58 inch; 

 convexity of the two valves united, 0.41 inch. 



The most marked feature of this species is its very strongly elevated, 

 sharp, irregular, concentric ridges that rise into larger angular folde 

 below the middle of the valves, thus giving an equally rugose appear- 

 ance to both valves. G. 2)eriin(lata, of M. and H., has nearly as strong 

 concentric ridges, but they are much more regular, and instead of being 

 shari>ly angular, with rounded, broader spaces between, as in the specie? 

 here described, they are broadly rounded, with narrow, angular furrow^s 

 between, on that sliell, which also wants the angular, posterior, umbonal 

 slopes of this species. 



Locality and position. — Eock Spring Station, Central Pacific Railroad. 

 Wyoming Territory. Upper part of Bitter Creek series. 



CORBULA TROPIDOPHORA, Meek. 



Shell distinctly trigonal, with height a little more than three-fourths 

 the length, moderately convex ; valves subequal, the right one being only 

 a little more convex than the other, and having its beak not much more 

 prominent; both with posterior umbonal slopes angular from the beaks 

 to the posterior basal extremity, that of the left valve especially being 

 very distinctly so, while the sides of the valves, (particularly that of the 

 left valve,) in front of the angular umbonal slope, are flattened ; anterior 

 outline truncated with slight obliquity from the beaks to the anterior 

 basal extremity, which is subangular or very abruptly rounded into the 

 base ; posterior dorsal margins abruptly inflected toward each other 

 from the umbonal ridge, and forming a nearly straight slope from the 

 beaks to the posterior basal extrejnity, which is distinctly angular; 

 pallial margin gently convex in outline from the front to the posterior 

 angle; beaks moderately prominent, oblique, incurved, contiguous, and 

 located about half way between the middle and the anterior margin. 

 Surface ornamented by fine lines of growth, and more or less defined 

 ridges and furrows that are usually most distinct on the lower half of 

 the right valve. Cartilage process of the left valve spoon-shaped, rather 

 prominent, and larger than the pit by its side for the reception of the 

 tooth of the other valve. (Hinge of right valve unknown.) 



Length of a medium-sized specimen, 0.72 inch ; height to top of beak 

 of right valve, 0.52 inch ; height to top of same in left valve, 0.49 inch ; 

 convexity of the two valves united, 0.33 inch. 



This shell is very closely allied to C. suhtrigo7iaH,s, M. and H., with 

 wh!.ch I have even been inclined to think it might be identical. The 

 tyjiical specimen of that species is a left valve only, so tliat we have not 

 the means of making a very thorough comparison. Com])ared with 

 this left valve, however, the corresjionding valve of the form under con- 

 sideration, although agreeing closely in general outline, is seen to be 

 decidedly more compressed, has its umbo less gibbous, and not so 



