384 GEOLOGICAL SURVEY OF THE TERRITORIES, 
Genus LIMNASA Lamarck 
LIMN 2A NITIDULA Meek. 
In some of the layers of the Bear River Laramie series, near the mouth 
of Surphur Creek, Bear River Valley, Wyoming, this Species occurs 
quite plentifully, where alone it has yet been discovered. It is described 
and figured in vol iv, U. 8S. Geol. Sur. 40th Parallel ae p. 181, 
plate ie figs. 5 and 5a. 
Subgenus PLEUROLIMNZA* Meek. 
LIMN aA (PLEUROLIMN AA) TENUICOSTATA Meek & Hayden. 
Dr. Hayden obtained the types of this species, which are also the 
types of the subgenus, from the Laramie strata near Fort Union, in the 
Upper Missouri River region, where alone it has ever been found. Mr. 
Meek established the subgenus for this species alone, no other having 
yet been discovered. It is described and figured in vol. ix, U. S. Geol. 
Sur, Terr. (4to ser.), p. 534, plate 44, figs. 13 a, b, and ¢. 
Genus ACKLLA Haldeman. 
Plate 30, figs. 9 a and b. 
Acella haldemani White, 1878, Bull. U. S. Geol. Sur. Terr., vol. iv., p. 714. 
Shell very small and very slender; spire longer than the aperture; 
volutions about six and very obliquely coiled, slightly convex; last 
one not very ventricose; aperture only slightly, if at all, expanded, its 
outer margin, as Shown by the lines of growth, being nearly parallel 
with the axis of the shell. Surface marked numerous fine linear raised 
lines corresponding with lines of growth with spaces between them about 
equal to the lines, their direction being nearly or quite parallel with the 
axis of the shell upon the volutions of the spire, as well as upon the 
body volution. These lines, owing to the minute size of the shell, are 
distinguishable only under a lens of considerable power. 
Length, 6 millimeters ; diameter of last volution, 14 millimeters. 
This form seems unmistakably to belong to Acella Haldeman. It is 
the only species of that genus that has yet been found fossil in Ameri- 
can strata of any age, if we except an undescribed form in the Green 
River Eocene Group of Wyoming; and is especially interesting as show- 
ing the early establishment of that type among the Limneide. Meek 
regarded Acella as only a subgenus under Limncea, and possibly he was 
right, but as the type has been so long established in nature, and the 
limits of genera are of very unequal distinctness and value, I prefer to 
regard if here as a full genus. 
Genus PHYSA Draparnaud. 
PHYSA FELIX White. 
Plate 22, fig. la. 
Physa felix White, 1878, Bull. U. S. Geol. Sur. Terr., vol. iv, p. 714. 
Shell large; body volution inflated, shouldered at the distal border 
by an abrupt rounding of that part from the outer surface, the shoul- 
i ee ‘35h Meek’s diagnosis of this subgenus, see vol. ix, U. S. Geol. Sur. Terr. (4 to 
ser.), p 
