154 Fossil Shells from the Colorado Desert. [ March, 
forms, to which authors have given many specific names, but 
which I regard as varieties only of one and the same species. 
The Physa (P. humerosa), also though a well marked form in cer- 
tain respects, I am inclined to place as simply a southern or warm 
climate variety of the widespread P. heterostropha Say, which in- 
habits an area extending from the Atlantic seaboard, to the Great 
Slave lake in the north, to the Pacific coast on the west, and is 
found also in Utah Lake. 
And here I would remark that I do not assert that the distri- 
bution of these several molluscan forms may not have been 
through other channels and by other lines; from the Sierra pér- 
haps, down its eastern flank or from some point or points in the 
coast range further to the east. 
There are other agencies and methods of distribution, which I 
will refer to briefly, before closing. The Axodonte, for instance, 
when young, sometimes attach themselves, by their valves, to 
the fins of fishes, and are carried to new localities in this 
way, as well as by being swept along by streams. Physa 
and Planorbis in their embryonic stage are contained in a soft 
and sticky mass of jelly; this gelatinous mass adheres to the 
legs and feathers of birds, especially aquatic species, and is car- 
ried often to great distances from the original habitat ; again tor- 
nadoes and water spouts act as distributing agents, taking up the 
waters of lakes and streams, and the chips and twigs which fre-- 
quently abound therein, and transporting the same to places many 
miles away, where they are deposited, sometimes, in other streams 
or ponds. It is quite common to find the sticky egg-mass as well 
as adult individuals of the fresh water snails, adhering to such 
objects. 
And now after a somewhat prolonged and perhaps tiresome 
journey over a rather wide region, pursuing such suggestions as. 
have been evolved from time to time during the progress of our 
inquiry, let us return to the place of beginning, where I will con- 
clude by repeating the noteworthy facts, perhaps without parallel 
elsewhere, namely the level of the Southern Pacific Railroad in 
the Colorado desert, as compared with that of the ocean. Here 
we have several miles of railway upon a plane nearly 200 feet be-. 
low the sea level, and locomotives supplied with fresh water from 
a well 240 feet below the level of the ocean; the latter, if not the 
deepest, being certainly one of the remarkable wells of the world. _ 
