WHITE] ANCESTRAL ORIGIN OF UNIONID& 83 
the rising of the land, not the lowering of the river. The cafion of 
Green river through the Uinta mountain range, and the Grand 
Cafion of the Colorado of the West through the Great Plateau, are 
cases of this kind. Still, some rivers have suffered vertical displace- 
ments in at least parts of their course. For example, the prolongation 
of the channel of some existing rivers of North America, is trace- 
able by soundings beneath sea-level, where they sank by subsidence 
of the continental border. If that border should be raised again 
such rivers and their faunas would come into their former posses- 
sions. At the beginning of the Tertiary period the Upper Mississippi 
and Ohio rivers emptied separately into the Gulf which then 
extended northward above the present confluence of the two rivers. 
That is, the whole of what is now the Lower Mississippi was then 
beneath sea-level. It has since been added to the upper portion of 
the great river system and stocked with its fauna. 
These, and many other similar facts show that rivers, once estab- 
lished, although often modified in extent by land elevations and 
subsidences, and changed in direction by the opening of new lake 
outlets, have been among the most persistent features of the earth’s 
surface. The lakes which occupied portions of the course of ancient 
rivers have all been obliterated; and doubtless also in rare cases 
some rivers or small river systems, with their molluscan faunas, have 
been wholly destroyed. The facts which have been stated, however, 
warrant the assumption that, as a rule, some portions of those an- 
cient rivers have preserved a continuous flow of fresh water to the 
present time. I do not doubt that at least some portions of the 
present Mississippi River system represent a continuous fluvatile 
flow from a time at least as remote as the Cretaceous period. Rain 
waters have always fallen upon the land ever since its first elevation 
above the sea, and a constant flow of drainage streams has been 
necessary to remove it. It is only by a constant flow that genetic 
lines of fresh water denizens could have been preserved ; and I there- 
fore assume that the Unione fauna of the Mississippi River system 
has in this way been, at least in part, genetically derived from the 
fossil faunas some of whose remains are figured on the accompany- 
ing plates. 
Some of the types of former fresh water denizens whose remains 
have been discovered are not found among living faunas, and it 
is therefore inferred that these were among the faunas of those 
rivers which failed entirely to preserve their continuity of flow 
through successive geological periods. For example, although Unio 
belliplicatus, which is represented by figures 4, 5 and 6, on plate 
