272 PROCEEDINGS OF THE NATIONAL MUSEUM. 
the sea at a sheltered point. The water became constantly shoaler, 
until at length the separation was complete. " The deposited material 
has steadily increased the distance between the gulf and the low bed 
of the desert, until now the division is marked by a narrow neck of 30 
or 40 miles of land but little raised above the level of the sea. 
To quote Professor Blake: If the alluvial deposits brought down 
by the Colorado River were removed, the gulf would How inward and 
again occupy its ancient bed. When the stupendous work done b}' 
the Colorado K.iver in cutting deep canyons along its course is consid- 
ered, it is easy to realize the vast ciuantity of detritus brought down 
and deposited by that industrious and mighty stream. 
The Colorado desert of California is only a portion of a much greater 
desert area, which extends on the easterly side of the river into western 
and southwestern Arizona, including the desert of the Gila, reaching 
for a long distance to the base of a mountain range in the Mexican 
State of Sonora, the Sierra del Nazareno {(), or spurs of that range 
outlying to the north, and on the westerly, south of the California 
boundary, an area of great extent reaches still farther to the south 
into Lower California, bordering on the gulf. 
Here, also, we find a small depressed basin, known as Lake Maquata, 
its northern end about 10 miles south of the United States boundary, 
between the Peninsular and Cocopah ranges of mountains. Its surface 
is doubtless below sea level, but the sediment deposited by the Colo- 
rado has created a ^^ermanent barrier between it and the Gulf of Cali- 
fornia. Millions of fresh-water snail and "clam" shells are strewn 
over the bed or along the former shores of the lake, sufficient evidence 
that it had once been filled with fresh water. ^ In Sonora it embraces a 
large and indefinite area, of which but little is known. The northerly 
portion of the desert, that is to say, the California section, contains 
approximately 6,000.000 acres, inchided in the boundaries of San Diego 
and Riverside counties. To the east and northeast of the San Bernar- 
dino range, in these counties, lying between the range and the Colorado 
River, extending to the higher desert levels to the north, there is a 
region of numerous dr}^ lakes and springs, the latter usually dry or 
intermittent, according to the seasonal rainfall. Of the desert region 
exterior to the California portion but little is known, and the Califor- 
nian area, so far as the Mollusca are concerned, has been only partially 
explored. The species we are considering have been found in manv 
places quite remote from the Colorado desert. 
At the north, ranging as far as Death Valley in Inyo Countv, thence 
northeasterly to the shores of Sevier Lake in Utah. Turning south- 
ward, the occurrence of the PaJudrfdr/na in the States of Durango and 
Michoacan, Mexico, the latter region 1,800 mdes south of the Colorado 
desert, may well be regarded as most extraordinarv, as well as the 
'C. R. Orcutt, in West American Scientist, 1891. 
