116 The American Naturalist. [February, 
mounds left to be taken possession of by the indigenous plants 
of the region, which are always, under such circumstances, of 
much ranker growth than is usual under other conditions. 
Although I have been a resident of Iowa for more than 
twenty-eight years, yet comparatively few of the mounds under 
consideration have been reared by these Rodents during that 
time. 
The question may arise: If nearly all these structures (which 
are very numerous) are the works of Geomys bursarius, why are 
there so very few of them in process of erection to-day. 
To this query, we conceive of but one or two satisfactory 
replies; and thisis: In the somewhat remote past, this species 
reached its culmination, or greatest expansion, in the regions 
under consideration ; and that to-day witnesses its final decline; 
or else, that for an indefinite time in the past, this animal was 
a resident here, the mounds witnessing to their long abode, 
and upon the advent of the white settlers and the breaking up 
of the wild prairie lands, its mound-building propensities, 
or habits, was, to a considerable extent, modified. 
Instead of erecting these great mounds to-day, this animal 
usually throws up little hillocks of earth, as described farther 
on in this paper. 
One instance of what I may call the early mound-build- 
ing propensity of this animal, was observed some years ago by 
the writer on his fathers farm near Rockford, Iowa. 
First there was thrown up a small hillock of the usual size 
and appearance; and this was added to year by year (appar- 
ently by the same animal) until a conical mound three and 
one half feet in height and perhaps five feet in diameter at the 
base, was reared; when for some cause the animal ceased to 
work here. 
In this instance several small mounds or hillocks were seen 
in the vicinity. 
In fig. 1 is shown a sketch of this conical mound as last 
seen by the writer. 
Some of the early writers on this subject have supposed that 
the great mounds of the prairies referred to, represent the ruins 
of ancient “muskrat houses” the builders of which existed 
