43 



importance to tbe present and future inhabitants of the country. Again^ 

 as these deposits, now occupying such different elevations in Wyoming 

 and Colorado, are evidently of brackish- water or estuary origin, and 

 consequently were originally formed at the same level (that of the sea), 

 their present ditferent elevations give us some idea of the great changes 

 in the physical features of this internal region of the continent that 

 have taken place in comparatively modern geological times. 



A species of bivalve, that I have described on one of the following 

 pages under the name Cyrenaf Holmesi, came from four miles north of 

 Golden City, Col., at the eastern base of the Eocky Mountains. Accord- 

 ing to Mr. Holmes's observations, the bed in which *this fossil occurs 

 holds a position about 500 feet above the horizon of the coal-beds mined 

 near there. The specimens of this shell brought in, although showing, in 

 «ome cases, the form and ornamentation of tlie species well enough, un- 

 fortunately ill no instance give any satisfactory information in regard to 

 its generic relations 5 and, as the species is new and distinct from all of 

 those we have yet had from the well-determined horizons of this region, 

 it gives little or no information in regard to the age of the rock in which 

 it was found, though it is most probably Tertiary. 



Anomta micronema. Meek. 



Shell of medium size, thin, orbicular, subovate, or somewhat irregular; 

 upper valve moderately convex, or more or less depressed, even nearly to 

 flatness in some cases ; beak very small, depressed, and nearly, but not 

 quite, marginal ; cardinal margin generally a little truncated and slightly 

 thickened ; surface ornamented by very fine, regular, often deflected, 

 radiating stria, and small, sometimes regularly-dis[)osed, concentric 

 marks of growth. (Under valve unknown.) 



Diameter of well-developed specimens generally about 1 inch. 



This species is quite abundant, and generally moderately well pre- 

 served at the locality. As usual with fossil species of the genus, pnly 

 upper valves were found. These show the muscular impressions to be 

 exactly as in true Anomia. 



Locality and position. — From a shaft sunk on the Kansas Pacific Rail- 

 road, two hundred miles east of Denver, Col., 45 feet below the surface, 

 from beds of the age of the Wyoming Bitter Creek coal-series. 



CORBECULA? (LepTESTHBS) PLANUMBONA, Meek. 



Shell attaining a moderately large size, rather thick and strong, 

 especially about the hinge of large specimens, generally of a short trans- 

 versely-oval or subelliptic form, but rather variable in outline, moder- 

 ately and evenly gibbous, the greatest convexity being in the central 

 region ; anterior margin prominently and rather narrowly rounded ; 

 posterior vertically subtruncated ; base forming a more or less nearly 

 semi-elliptic or semi-ovate curve ; dorsal outline sloping from the beaks, 

 the anterior slope being more abrupt and concave in outline, while the 

 posterior is generally convex; umbones subcentral, moderately' promi- 

 nent or somewhat depressed, usually eroded, and more or less flattened 

 near the apices, which are not strongly incurved, distinctly pointed, or 

 raised much above the hinge-margin ; lunular region, in the specimens, 

 with more gibbous umbones, somewhat excavated, but not distinctly 

 impressed, or with defined margins ; ligament narrow and not very 

 prominent; anterior muscular impression ovate, well defined, and dis- 

 tinct from the small pedal scar under the hinge, above and behind its 



