WHITE] ANCESTRAL ORIGIN OF UNIONID/E 81 
given by these illustrations is of a general character, but one who is 
familiar with the living fauna of the Mississippi River will not fail 
to recognize a close similarity of some of its members to certain of 
the fossil species. Full artificial expression of the general relation- 
ship that exists between these fossil species and those which are now 
living in the Mississippi River system would require a large number 
of figures of the living, as well as of the fossil, species. As such a 
full illustration is, for obvious reasons, not now practicable, the 
reader is referred to the publications mentioned below’ or, better 
still, to the mollusks themselves in their native waters. 
Before proceeding with special references to the figures upon the 
accompanying plates and to the fossil species which they represent, 
some explanation of relevant paleontological and geological facts in 
their relation to ancient physical geography is necessary. For the 
sake of brevity these explanatory remarks are mostly made in sen- 
tential, rather than in strictly consecutive, form. 
Fossil shells of the Unionidz have been discovered in great num- 
bers and variety in many parts of the world and in formations of 
various geological periods. They are found imbedded in more or 
less hardened rocky strata that originally consisted of muddy or 
sandy sediment at the bottom of bodies of fresh water. Those 
lacustrine waters were finally shifted to other areas by oscillations 
of land surface or drained away by the deepening of the channels of 
outlet, but they left an unmistakable record of their fresh-water 
character in their fossiliferous sediments, which remained. So re- 
stricted are the living Unionide to fresh waters, and so distinctive 
are the shell characters and the shell texture of all the members of 
the family, that the geologist is as certain that the strata containing 
their fossil remains were deposited in fresh, and not in marine, 
waters as if he had then been there and analyzed them. Moreover, 
there are usually found with the fossil shells of the Unionide the 
shells of other mollusks which are similar to those of the associates of 
their living congeners. 
The existence of a lake, or a body of fresh water, implies the 
coexistence of a surrounding land surface upon which flow drainage 
streams of inlet and outlet. The existence of a stratified deposit or 
formation containing remains of fresh water mollusks implies that the 
* Observations on the Genus Unio. By Isaac Lea. Vols. xin quarto. 
Profusely illustrated by full-page plates, part of which are colored. 
Synopsis of the Naiades, or pearly fresh-water Mussels. By Charles Tor- 
rey Simpson. Proc. U. S. National Museum, vol. xxu, pp. 501-1044, and 
plate xvi. The literature of the Unionide is very extensive. That for 
North America is catalogued by Mr. Simpson in the forementioned work. 
