382 ©. A. White—Fresh-water gill-hearing Mollusks. 
ao aces On Certain conditions attending the Geological 
De: of some North American iypes of Fresh - water gill- 
vias Mollusks ; by C. A. WHITE 
[Extracted in advance ei the Annual Report of the U.S. Geological Survey for 
y permission of the Directo r.] 
THE following see are extracted from an article entitled 
Mollusca of the Laramie and Eocene deposits of Western North 
America, and their descendants now living in the reve 
drainage- -system, and are offered as an explanatio on of the 
ner in which that system became stocked with its present c shine! 
oe Aye paw aia and doubtless to a large extent, with 
ts ichthyic fauna 
When we store to trace the probable lines of succession 
or descent of the various types of Mollusca that now exist, the 
difficulty seems especially trea when casually considered, i in 
the case of the fresh-water gill- bearing Mollusca. 
The prevalence of the sea has always been practically uni- 
versal ; and the various movements which the earth’s crust has 
undergone since life began in the sea, while they have repeat- 
edly distarbed or destroyed the habitats of its molluscan deni- 
zens in certain localities, and many o “on lines of genetic suc- 
cession of types that had from time to time become establish 
have been broken, there has e ony never been anything 
like such a general destruction of life in the sea as would either 
break or materially interfere with the greater aol ‘of the prin- 
oe lines of such succession. in short, the ne field for 
e development and perpetuity of hpHeMoste ‘life has been 
ae and unbroken from the beginning to the present time, 
and we are at no Joss to understand how continuous lines of 
genetic succession of its denizens may have extended down 
through all the geological ages, ipsa it is true, by immedi- 
ately environing and cosmical causes, but still u nbro ken. We 
may at least conclude that if ay ‘olfosean 7 sess that now 
exists in the sea bas not been lineally m the ae 
molluscan forms that have e sisted in it hee th ave been 
such changes of its physical Seaton: as would preclude ae 
a possibility. 
When we come to the study of the fossil Pepaianee Mollusca, 
especially the land-shells, we have le or ifficulty 
in understanding how it hus been ‘pgs for contin lines 
of existence of these mollusks to be preserved through succes- 
sive geologcial periods upon any esac area, “such for - 
example as North America, notwithstanding the numerous and 
