1876] 
To and its Habits. 325 
is not at this time a subject of inquiry. In color, the shells of 
French Broad River are remarkable for green tints in the epi- 
dermis. In the Holston River the tints are somewhat ferruginous, 
but not to the same extent observable in the shells of Powell’s 
River ; while in Clinch River the epidermis is often of a bright 
yellow or orange, varied by livid tints which are partaken of by 
many other univalve mollusks inhabiting that stream. 
The reader may possibly have felt, in reading a portion of this 
paper, some curiosity to know why Io occurs only in the upper 
waters of the Tennessee system of drainage. In the introductory 
paragraph of this paper it was stated that “ all these streams, not 
excepting the upper portions of the Tennessee River itself, have 
in a greater or less degree the characters of mountain torrents, 
which in reality they seem to be on a very grand scale.” m 
what we now know of Io we may infer that it cannot exist in 
placid rivers, and the limit of its distribution south in the Tennes- 
see River depends on the character of the river. At the point 
where the Tennessee begins to be a majestic, placid stream, there 
To ceases to extend its domain. This very simple inference is a 
key to the solution of other problems relating to the geographical 
distribution of allied forms in the same great family of mollusks. 
Very many of the univalve mollusks of the Tennessee drainage 
abound in swift shoal water, among rocks over which the water 
flows in broken torrents, and nowhere among still waters. The 
Tennessee River at Mussel Shoals is very prolific in various 
forms of mollusk life which delight in a rapid current ; but below 
that point, in the navigable portions of the stream, very many of 
these interesting species disappear, because the conditions are no 
longer favorable to their existence. i 
Thus far, in the main, only the typical To has been considered. 
There is another group of shells very nearly allied to Io, known 
by the generic designation Angitrema. Mr. Reeve regarded these 
shells as properly belonging in the genus Io; but his views do not 
Seem to have met with much favor by writers on American 
conchology on this side of the Atlantic. Notwithstanding this 
evident difference in opinion, there is much reason for believing 
that Mr. Reeve’s position is a good one, for some of the species of 
Angitrema are apparently related to Elk River, the Cumberland 
River, and some of its tributaries, as Io is to the head-waters of 
the Tennessee River. Indeed, it appears to be true that near the 
Point where To begins to disappear in the Tennessee River in 
Alabama, some of the forms of Angitrema replace it. The ques- 
