1876.] To and its Habits. 323 
them must have come over the sea. It does not appear that 
they [Ios] had been observed in their native element, though liv- 
ing at the very doors of the persons who had remarked them in 
the tumuli.” The impression that Io is a “ sea-shell ” is one that 
strikes most ordinary observers at first sight, as every collector 
who has them can testify. 
Quite a considerable number of species of Io may be found 
described and figured in various works treating on the shells of 
North America, and there is considerable diversity exhibited in 
their forms, sizes, colors, and markings. There seems to be con- 
siderable difference of opinion among persons who have studied 
these shells, as regards the number of species. There are some 
individuals who with apparent good reason believe that there is 
really only one species of typical Io, to which all the forms are 
subordinate as varieties; while on the other hand we shall find 
others who for reasons quite as good insist that there are five or 
more “ good species.” The shells, taken by themselves, without 
regard to any facts relating to their habits, do not afford conelu- 
sive testimony as to species, as it is exceedingly difficult to isolate 
forms that cannot be made part of a continuous series when large 
numbers of shells are placed together. There are, however, some 
facts connected with geographical distribution and the association 
of varied forms, which seem to indicate conclusively that there 
are certainly two species; and, this being admitted, the logical 
inference might be, under all the circumstances which remain un- 
Considered, that there are more than two species. This, however, 
18 a question which remains to be investigated. The facts upon 
Which two Species are inferred are the following. At Black’s 
Ford, Clinch River, Anderson County, Tenn., two forms of Io con- 
stitute very nearly all that are found at that particular locality, 
and these two forms occur there in about equal numbers, and 
there are not associated with them any intermediate forms uniting 
the extremes. Thirty or forty miles down the river (following 
its winding course), at Williams’ Ford in Roane County, these 
Same two forms reappear, but their relative numbers have changed. 
At this point the form which seems to be identical with a shell 
figured by Reeve as Jo turrita (not Mr. Anthony’s species of that 
name, by any means) is the shell occurring in fewest numbers, 
while the prevailing form is a graceful, slender variety of the shell 
that Mr. Lea calls spinosa. The change in the relative abundance 
= two forms by a change of station seems to afford evidence rela- 
e to species. In following out the train of ideas which this 
