WHITE. ] LARAMIE FOSSILS. 81 
obvious; beaks moderately prominent, that of the left valve being 
only a little more elevated than that of the other, located in advance 
of the middle, contiguous, incurved, with a scarcely perceptible, forward 
inclination ; posterior extremity subangular at the connection of its 
margin with the base; posterior dorsal slope more or less convex in 
outline; anterior margin rather short and rounded; base semi-ovate in 
outline, being most prominent anteriorly, and somewhat straightened 
behind; left valve about one-fourth less convex than the other, with 
posterior umbonal slope less strongly angulated ; surface of both valves 
ornamented with concentric ridges, eenerally small and regular on the 
umbonal region, but often swelling out into a few very prominent angu- 
lar 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 eenerally becoming obsolete behind the angular poste- 
rior umbonal slope, but continued forwar d to the front. 
“Length of a well-developed specimen, 0.76 inch; height to top of 
umbo of left valve, 0.56 inch; convexity of the two valves united, 0.41 
inch. 
“ Locality and position.—Rock Springs Station, Central [ Union] Pacific 
Railroad, Wyoming Territory. Upper part of the Bitter Creek series.” 
Typical forms of this species have been found in the valley of White 
River, Northwestern Colorado, near the base of the Laramie Group there, 
which shows it to range nearly or quite through the whole thickness of 
the group. Figs. 4a and b, on plate 29, are of an unusually large ex- 
ample, from Rock Springs. 
CORBULA UNDIFERA, var. SUBUNDIFERA. 
Plate 29, figs. 5 a, b, and c. 
Corbula subundifera White, 1876, Powell’s Rep. Geol. Uinta Mts., p. 129. 
This form was originally described by me as a distinct species, but it 
is So very nearly related to C. undifera, the form just described, that I 
am now disposed to regard it as only a variety of that species. I how- 
ever give it a separate description and illustration in this article, be- 
cause it is so constant in the minor differences which distinguish it 
from the typical form, and because those differences are recognizable at 
widely separated localities, examples of this variety having been brought 
by Professor Powell from Upper Kanab, Southern Utah. 
Shell of ordinary size; marginal outline subtrihedral or subovate ; 
valves only slightly unequal; beaks contiguous; umbones moderately 
prominent; beaks incurved and directed a little forward; front obliquely 
truncate, concave, producing indistinctly defined anterior umbonal ridges; 
abruptly rounded below to the basal margin, which is broadiy rounded ; 
posterior extremity low, prominent, and shar ply rounded; postero-dor sal 
margin sloping from the dorsum to the posterior extremity; this margin 
of each valve is bent abruptly inward and downward, pr oducing a nar- 
row, shallow furrow, bordered at each side by a somewhat prominent 
ridge, which extends from behind the beak to the posterior extremity of 
the shell. Surface marked by numerous more or less strongly elevated 
concentric folds, which disappear before reaching the posterior margin, 
and are usually less distinct in front than upon the sides. Between 
these folds, and upon those parts of the surface unmarked by them, 
there are ordinarily distinct concentric lines of growth. 
Length, 25 millimeters; height, 18 millimeters. 
Position and localit y.— With ‘the exception of the Southern Utah spec- 
6H 
