274 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiv. 
While the winds and waters have no doubt played an important part 
in the dispersion of these and related molluscan forms, we are not 
restricted to these agencies to explain their widespread distribution. 
At the present day we find numerous species of aquatic birds fre- 
ciuenting- the springs, pools, and marshes or wet meadows wherever 
such places occur within the larger or general desert area, both north 
and south. When the present deserts were lakes or lagoons, whose 
waters covered several hundreds of square miles, and the numerous 
springs, now dry or intermittent, filled their basins to overflowing, the 
number of swimming, diving, and wading birds throughout the region 
must have been very great. In their seasonal migrations or ordinary 
flights from lake to spring or spring to lake they would unavoidably 
have carried from one place to another such indiv^idual mollusks as 
adhered to their legs or feathers, and the sticky Q,g^ masses of these pond 
snails also; it is not unreasonable to assume that these were frequently 
transported in this wav over considerable or even great distances. 
It should be borne in mind that these tiny mollusks live in the midst 
of the conferva, and that the best way to obtain specimens is to col- 
lect a large quantit}^ of this green scum-like material and spiead it on 
paper in the sun, when it quickly dries up and the shells are easily 
shaken out. The carrying of these little forms from place to place is 
facilitated by the character of this vegetable growth which they fre- 
quent, as a comparatively small quantity, a little bunch of it, entan- 
gled in the feet or on the legs or among the feathers of a duck might 
contain many individuals. 
That these minute forms {Paludestrince) are still living in many other 
localities in the Great Desert than the few which we now know of 
seems altogether probable when we consider their inconspicuous size 
and that their shells are usually coated over with the confervoid 
growth that is generall}^ present in and around springs. Being 
immersed in this green vegetable scum, they escape detection and are 
not likely to be seen by persons not familiar with their habits or not 
especially interested in searching for them. Wherever they do occur 
they compensate by vast numbers for lack of size, for these little 
fellows are wonderfully prolific.^ 
The evidence of lake or lagoon conditions resulting from the over- 
flow of the Colorado River in the past, is corroborated by the perpen- 
^ Some idea of the immense number of these shells on the surface of the desert may 
be derived from the following: " We soon noticed that the soil was, to a large extent, 
made up of minute turreted shells, so small that they could hardly be distinguished 
from the grains of sand. At certain points they were blown by the wind into wind- 
rows, and lay in vast numbers, concentrated from the sands by a natural winnow- 
ing process. These are the * * * fossils described in the Smith. Misc. Colls., 
No. 144, part 3, folio 70. * * * On my return to San Francisco I made experi- 
ments showing that it required 166,000 of these minute shells to weigh a {)ound." 
H. G. Hanks in Second Report of State Mineralogist of California, 1880-1882, 
appendix, i)p. 227-228. 
