148 Fossil Shells from the Colorado Desert. [ March, 
An examination of Lieut. Wheeler’s Drainage Map! of the ter- 
ritory west of the one hundredth meridian, shows the position of 
the Colorado as related to the other basins, to be diagonal, its 
general trend being north-easterly and south-westerly. 
“The region of country drained by the Colorado and its tribu- 
taries is about 800 miles in length and varies from 300 to 500 
miles in width, containing about 300,000 square miles, an area 
larger than all the New England and Middle States, with Mary- 
land and Virginia added, or as large as Minnesota, Wisconsin, 
Iowa, Illinois and Missouri.’ 
While it may be presumed that these forms so intimately re- 
lated have gradually through a great period of time spread from 
the higher altitudes to the lower, from the older formations to the 
more recent, extending the area of their distribution as fast as the 
emergence of the land above the sea and the emerged sur- 
face had undergone those modifications necessary to fit it for their 
occupancy, and are still spreading to new localities; so perhaps in 
other portions of the area of their domain they are decreasing or 
dying out, or have already died out, through modifications in the 
environment as in the now dried up lake or lagoons of the 
Colorado desert. 
It is highly probable that at numerous places within the great 
interior basin, after the emergence of the land, fresh water lakes, 
ponds and streams occurred and existed for a time, and be- 
came peopled, in part by fluviatile species, such as we are now 
considering, including varieties of the mussels, intermediate and 
connecting links between the desert specimens and those from the 
Wahsatch stations referred to. 
It is to be expected that in course of time when this great in- 
terior wilderness is thoroughly explored, that living specimens 
‘will also be found in those streams and lakes whose waters are 
still sweet ; and with a sufficient number of specimens living and 
fossil from these two classes of stations, we may be able to trace 
the lines of distribution, which followed not alone the direct 
course of streams, for instance like the Colorado, but descended 
also to the great basin from the now lofty altitudes of the Sierra Ne- _ 
vada, and the parallel ranges on its eastern flank on one side, as 
from the corresponding regions in the Wahsatch range and sub- 
1In Lieut. Wheeler’s Annual Report, 1876. 
= #Exploration of the C the West, etc., by Prof. J. W. Powell, 1875, P. L 
