1879. | Fossil Shells from the Colorado Desert. 147 
has been permanently and continuously dry ever since the extinc- 
tion of Tryonia, but that through evaporation, ¢¢ ceased to be per- 
manently covered with water, and though since at various times, a 
greater or less area has been so covered—the extinction of Tryonia 
proves that there must have been a period of drouth, a maximum 
both as regards degree and duration which destroyed the limited 
fauna, the few species which once existed here—though some of 
the species which I have named belonging to the genera, Axo- 
donta, Physa, and Planorbis, having an earlier genesis, are known 
to have a wider distribution, and migrated hither, as they are still 
found living in regions more or less distant, as will be shown 
farther on; and it is barely possible that a living colony, a rem- 
nant of Tryonia may yet be discovered somewhere within the 
desert area. : 
It has been implied herein that the metropolis of the form 
called Zryonia was here in the former lake or lagoon region of 
the Colorado desert or basin, for the reason of the great multitude 
of individuals; its specific center may have been far to the north, 
nearer to the point from which the Wheeler specimens, referred 
to by Mr. Tryon, were obtained; it has not been found and is not 
known to be living elsewhere, at the present time. As to the 
other molluscan forms which once lived here and are now found 
dead either at or below the surface, an inquiry as to the lo- 
calities where they are still found living,’ and the probable paths 
of distribution which brought them together as congeners or 
Amnicola longinqua Gould (showing range of variation). Drawn by Stearns from 
recent specimens collected by Henry Hemphill, in Utah Territory. 
neighbors of Tryonia, incidentally furnishes further informatién 
pertaining to the specific region formerly occupied by these 
species. — 
_ 1Amnicola longinqua, has recently been collected ving in Utah by Mr. Hemp- 
