The shell is very small, perforate, broadly ovate, corneous,
translucent, thin; surface glossy, very minutely marked with
delicate growth-lines. The outlines of the spire are convex,
the apex somewhat pointed. Whorls 3½, strongly convex, the
last more rapidly descending close to the aperture. The aperture is ovate, somewhat oblique, angular above. Peristome
continuous and free from the preceding whorl. Length 2.4,
diam. 1.7 mm.; longest axis of aperture 1.25 mm. Length 2.2,
diam. 1.6 mm.
Washington Co., Utah. Types no. 121,112, Wheatley collection, in coll. A. N. S. P.
This little shell resembles the larger Bythinella palomasensis,
from Lake Palomas in northern Chihuahua (NAUTILUS IX, 68,
Oct., 1895; Dall, Proc. U. S. Nat. Mus. XIX, 1897, p. 369, pi.
31, fig. 9). The present species is broader, and evidently old
individuals are smaller. The generic position is uncertain. As
between a short Paludestrina and a long Amnicola there is little
choice.
Washington county is in the southwestern angle of Utah,
drained by the Virgin river, flowing into the Colorado. The
specimens are "dead" shells, but not fossil, I think. According to the label, Oreohelix strigosa was found in the same place.
The collector was not given for this or the preceding.
All of the adult specimens of A. deserta have the last whorl
shortly free at the aperture. It is a senile form, probably extinct or on the verge of extinction, Amnicolidae lead a precarious
existence in the arid states. The rivers do not afford suitable
stations. They have apparently never gained access to the
small perennial streams of the higher mountains; and permanent
springs and streams are so rare on the lower levels that the
colonies are small, few and widely separated. The large proportion of extremely diminutive species in the arid region is
remarkable. It may, perhaps, be looked upon as a permanent
dwarfing due to unfavorable conditions.