break through in stepping on a mound, since the whole is a honeycomblike
structure of from two to four stories in vertical plan, as shown by the
transect of a mound in Plate VII, Figure 1. As Bailey writes, these
partition walls are a mixture of earth and old food and nest material
discarded years ago, resembling the adobe walls of the Mexican houses
built of chopped earth and straw. This is the result of the continual
ejection of refuse and earth as before mentioned, combined with the
caving action of rains and disturbances from larger animals.

Apparently there are no special pockets for deposit of feces in
_Dipodomys_ burrows; such matter may be found throughout the den, and is
more or less mixed with the food refuse which carpets practically the
entire tunnel system. The nest and food stores are, however, clean and
neat, the droppings being dry and, though present on the floor of a
storage chamber, not actually mingled with the food. Evidently the
animal does not clean up the floor litter before storing food material.

The entire system for any one den seems to consist not only of the
burrows within the mound itself, as described, but of those small
outlying ones which we have referred to as subsidiary burrows. These are
two to four in number, and are connected with the main mound by the
runways already mentioned. They often seem to be way stations on the
runways connecting main mounds, and there is seldom any mound of earth
whatever in connection with them. One entire den system, the home mound
and three subsidiaries, was mapped after being excavated (Fig. 3), all
having been carefully gassed with carbon bisulphide. The subsidiaries
were simple and contained no storage. Two of them were shallow, while in
the third a depth of 48 centimeters was reached. They appear to be
merely places of refuge, though the well-worn trails connecting them
with the main mound indicate regular use. These runways are conspicuous