WHITE. ] LARAMIE FOSSILS. 59 
conspicuous only because it increases the apparent concavity of the 
basal portion of the valve. Surface marked by numerous, rather coarse. 
radiating lines or small cost, which increase in size towards the free 
margins of the shell. These cost have generally a somewhat crenu- 
lated aspect, which is due, in part, at least, to small sinuosities in their 
course, and in part to being frequently crossed by lines and undulations 
of growth; denticles or crenulations of the short front margin distinct, 
but not large. 
Length of the type specimen, 36 millimeters; breadth at the widest 
part, 18 millimeters ; but several less perfect examples which have been 
obtained at different localities indicate a much larger maximum size, 
the largest of which must have had a length of about 65 millimeters. 
Position and locality.—The type specimen is from near the base of the 
Laramie Group in the valley of Crow Creek, Northern Colorado, about 
fifteen miles above the contiuence of that creek with Platte River. Other 
examples are from Cation Park, valley of Yampa River; Danforth Hills, 
near White River Indian Agency, Northwestern Colorado; and Rock 
Springs Station, Union Pacific Railroad, in the valley of Bitter Creek, 
Wyoming. 
VOLSELLA (BRACHYDONTES) LATICOSTATA White. 
Plate 25, fig. 4 a. 
Volselia (Brachydontes) laticostata White, 1878, Bull. U. S. Geol. Sur. Terr., vol. vi, 
p- 708. 
Shell transversely elongate, arcuate-subelliptical; upper border broadly 
and almost regularly arched; posterior border somewhat abruptly, but 
continuously rounded from the upper border to that of the base, which 
latter border is gently concave along its middle portion; front abrubtly 
rounded; beaks inconspicuous, situated near the front; hinge line short, 
nearly straight, not forming an angle with the remainder of the upper 
border; denticles or crenulations of the anterior border distinct but not 
large. Surface marked by the usual distinct lines of growth and also 
by fine radiating coste, which are obsolete along the whole length of 
the median portion of each valve, and are more distinct upon and near 
the dorsal border than elsewhere. 
Length, 50 millimeters; greatest width, 19 millimeters. 
This species differs conspicuously from V. (B.) regularis, in having a 
greater proportionate length, straighter and less crenulate cost, and 
in the absence or obsolescence of the latter upon the middle portion of 
the valves. : 
Position and locality.—Laramie strata, about four hundred feet from 
the base of the group; Danforth Hills, near White River Indian Agency, 
Northwestern Colorado. 
Genus AXIN AVA Poli. 
AXIN/A HOLMESIANA White. 
Plate 20, figs. 2 a and b. 
Shell subeireular, the height and width being nearly equal; ligament 
area of moderate size, well defined, and marked upon each side of its 
median line by about seven distinctly divaricating linear grooves; beaks 
moderately gibbous, obtuse; hinge somewhat strongly arched; lateral 
teeth six or seven in number on each side, moderately strong, straight 
