58 GEOLOGICAL SURVEY OF THE TERRITORIES. 
lar; upper valve moderately convex, more or less 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 striz, and small, sometimes regularly disposed con- 
centric marks of growth. Under valve unknown. 
‘Diameter of well developed specimens, generally about one inch. 
‘This species is quite abundant, and generally moderately well pre- 
served at the locality. As usual with fossil species of the genus, only 
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 
Railroad, two hundred miles east of Denver, Colo., 45 feet below the 
surface, from beds of the age of the Wyoming Bitter Creek Coal series.” 
This Anomia has proved to be one of the most widely dispersed and 
abundant species of all that pertain to the Laramie Group, being 
especially common in the valleys of Bitter Creek and Yampa River, 
west of the Rocky Mountains; and also in the various fossil localities 
of that group east of the Rocky Mountains in Colorado. It is also 
known to range throughout the whole thickness of the group, a maxi- 
mum of at least three thousand feet. The species shows not only the 
usual great variation of form common to all Anomias, but also as regards 
the character of the radiating lines which mark the surface. In some 
cases, even in well-preserved examples, the radiating lines are obsolete 
or wanting. Others have those lines much coarser than the average; 
and the examples thus varied are found associated with an abundance 
of those which are normally marked. The examples which are figured 
on plate 25 have been selected for the purpose of showing this variation 
of surface markings. Considerable difference in average size and in 
general form has also been observed at different localities, which differ- 
ences were doubtless due to certain different conditions of environment 
while the mollusks were living. Thus, while at all the known localities 
of Laramie fossils east of the Rocky Mountains there is much variation 
as to the surface markings, there is comparatively little variation as to 
adult size and general form. But in the valleys of Yampa and White 
Rivers, west of those mountains, the specimens are generally much 
smaller; and in the strata at Rock Springs, in the valley of Bitter Creek, 
while the specimens are generally of the usual size, they have more than 
the average convexity of those found elsewhere. All these differences 
are, however, believed to be only varietal modifications of one species. 
Genus VOLSELLA Scopoli. 
Subgenus BRACHYDONTES Swainson. 
VOLSELLA (BRACHYDONTES) REGULARIS White. 
Plate 25, fig. 3 a. 
Volsella (Brachydontes) regularis White, 1878, Bull. U.S. Geol. Sur. Terr., vol. iv, p. 707. 
Shell arcuate-subovate in marginal outline; valves moderately con- 
vex; upper margin more or less strongly arched from the beak to the 
posterior portion; thence, with a continuous but stronger curve to the 
postero-basal margin, which is somewhat abruptly rounded to the gently 
concave base; front moderately narrow, slightly projecting beyond the 
beaks, and abruptly rounded to the base; beaks depressed, scarcely 
perceptible as such, and nearly, but not quite, terminal; hinge margin 
Short, nearly straight; umbonal slope somewhat prominent, but it is 
