84 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
XXVIII, has all the structural and textural characteristics of the genus 
_ Unio, it is not only the earliest known species of that genus to 
possess well marked surface ornamentation, but its type of orna- 
mentation is different from that of any known living North Amer- 
ican species. Besides this, several species of the gasteropod mollusks 
which are associated with this Unio are also different in certain 
characteristics from any of their kind upon this continent, either 
fossil or living. Moreover the Bear River formation, in which this 
fossil fauna is found, is of small extent compared with the other 
North American fresh water formations. From all these facts I 
infer that the body of water in which the Bear River beds were 
deposited, together with its inlets and outlet, constituted a small 
separate river system with a distinctive fauna. Also that its case 
was an exception to the rule of the persistence of the rivers, and 
that this whole small river system with its fauna became destroyed 
by some geological disturbance of the land surface. The types of 
the Bear River fauna which were not thus destroyed, for example, 
the simple type of Unio nucalis, which existed before, and have 
existed ever since the Bear River epoch, were probably preserved in 
other bodies of fresh water by collateral lines from an original 
genetic source. These remarks upon ancient physical geography 
may be closed with the following summary statements, together with 
references to the figures upon the accompanying plates and to the 
species which they represent. 
Fresh-water gill-bearing faunas have as certainly descended 
genetically through successive geological ages to the present time 
as have marine faunas. The genetic successors of each fauna have 
necessarily descended in a continuous fresh-water habitat. Such 
continuity of habitat has been produced and preserved by the sea- 
sonal rains which have always fallen upon the land and caused 
a constant drainage flow in its rivers and their branches. There 
has never been any intermission of such continuity because the fresh 
water supply has never failed, and because, as a rule, rivers have 
been among the most persistent of the earth’s surface features. 
While some rivers, or small river systems, have doubtless been from 
time to time destroyed by certain special movements of the earth’s 
crust and their peculiar faunas utterly exterminated, it is not prob- 
able that through all the great vicissitudes of continental devel- 
ment any greater proportion of fresh-water types have been thus 
destroyed than of marine types which have perished by volcanic 
eruptions, local elevation or depression of sea-bottom, changes of 
sea-currents, and other causes. 
