. 
208 A. G. Wetherby— Distribution of Fresh-water Mollusks, 
and published descriptions of the soft parts of 254 species of 
this family, and have often dissected 50 to 100 of the same 
species, I-can not see, as yet, any useful division that cou ei 
satisfy the student or the adept, which can be made by syst 
atic difference in the organic forms of the soft parts.” This 
means, I suppose, that the differences of the soft parts are so 
small as to afford no safe -basis upon which to predicate class- 
ification. I may add to this, that the most intimate study of 
the anatomy of different species of the Limneide and Strepoma- 
tide, has convinced me beyond reasonable doubt, that specific 
differences, supposed to be indicated in the shells, do not ex- 
tend to the animals themselves, so far as these studies go to 
show. I have now in course of preparation a memoir on this 
subject, which I hope soon to publish with accurate anatomical 
illustrations. Here is one of those strange facts, standing at 
the very threshold of the question of evolution, which finds 
a ee in the Zingula and the ee od 
ay now venture upon a few s estions, to which these 
the continent would have earally inhabited Archzean regions ; 
and as it is altogether likely, from chemical facts associated 
with the deposit of iron ores, ai the presence of graphite in 
the older rocks of the continent, as a out by Dr. Hunt 
and Dr. Dawson, that organic life may have existed to an 
extent not yet determined by rani: coil discovered as 
such, I think we do not pass beyond the bounds of probability 
in assigning to Fauna A a very remote antiquity. From its 
original locus, it has spread to the limit of suitable conditions, 
a limit undergoing constant variations, perhaps, through the 
geological ages, but which has been determined by boundaries 
mainly fixed by true geological causes. Through adaptation 
this fauna has, in a few cases, overstepped its primitive barriers, 
but it remains, as we have seen, true to its original instincts in 
all its more important phases. It is not probable, as may be 
suggested by the doubting reader, that this fauna wouls ae 
en exterminated by the great glacier, which is osed t 
have originated in its peculiar haunts, but more likely that Pe 
few species having an abnormal southern or southwestern range, 
received the first impulse of distribution in that direction from 
the glacial condition; and that, with the northward retreat of 
the glacier, they simply resumed their normal habitat, continu- 
ing their Gesribunens in that direction, in sueceeding times, to 
the northern lakes of British America. In st udying Fauna B, 
we find evidence that a previous distribution, probably sev- 
ered by the same or other causes, has never been fully united 
in a few cases—as in that of the MZ. margaritifera, occurring in 
Maine and Oregon, but not between these stations so far as now 
