98 GEOLOGICAL SURVEY OF THE TERRITORIES. 
From the former I obtained H. recta White, and from the latter Pro- 
fessor Powell brought H. utahensis White. I have very little doubt that 
the strata of both those localities really belong to the Laramie Group, 
but, as before explained, they are not specially considered in this article, 
because more information concerning their stratigraphical relations is 
needed. 
Genus MICROPYRGUS* Meek. 
MICROPYRGUS MINUTULUS Meek & Hayden. 
This little shell is the type and only certainly known species of the 
genus. It was obtained by Dr. Hayden from the Laramie strata near 
Fort Union, on the Upper Missouri River, and is deseribed and figured 
in vol. ix, U. S. Geol. Sur. Terr. (4to ser.), p. 575, plate 43, figs. 18 a and 
b. Ihave seen fragments of a shell in the Green River Group of South- 
ern Wyoming that may perhaps prove to be another, and considerably 
larger, species of this genus. 
Genus VIVIPARUS . Montfort. 
VIVIPARUS PLICAPRESSUS White. 
Plate 28, figs. 3 a and D. 
Viviparus plicapressus White, 1876; Powell’s Rep. Geol. Uinta Mts., p. 183 
Shell rather under medium size; spire conical, its sides eee straight ; 
_volutions about seven, moderately convex; the outer and anterior con- 
vexity of the last volution continuous and uniform; suture impressed. 
At the distal border of each volution there is a small, more or less dis- 
tinct revolving groove or furrow; and the narrow border between the 
furrow and the suture is in the form of a fold, which is closely appressed 
against the proximal side of the adjacent volution, the fold forming a 
shght projection or shoulder upon the proximal side of and bordering 
the suture. Surface marked by the ordinary lines of growth, and upon 
some examples there appears to be a faintly raised revolving line, or in- 
cipient angulation, near the middle of the outer side of the volutions. 
None of the examples in the collection are entire, but the length is 
estimated from the most perfect examples at about 25 millimeters; 
breadth of last volution, 12 millimeters. 
Position and locality.—Laramie Group, Black Buttes Station, Union 
Pacific Railroad, Wyoming; and also in the valley of Yampa Itiver, 
near Canon Park, Northwestern Colorado. 
VIVIPARUS PRUDENTIUS White. 
Plate 28, figs. 5a and b. 
Viviparus prudentia White, 1878, Bull. U. 8. Geol. Sur. Terr., vol. iv, p. 716. 
Shell depressed, subconical ; spire short, convex; volutions five and 
a half or six, including the minute ones of the apex, a all regularly con- 
vex; last one. considerably enlarged, and constituting the greater part 
‘of the shell, rounded by an almost uniform curve from the suture to the 
umbilicus ; ‘suture well defined, its apparent depth increased by the bold 
convexity ’of the volutions ; umbilicus very small and deep; aperture 
‘short, subovate or subcircular in outline; obtusely angular at its distal 
* For diagnosis of this genus see vol. ix, U. 8. Geol. Sur. Ter., (4to ser.,) p. 574. 
