J. Memmler 1943 Perognathus californicus June 12, m.v.z. they started at once to dig underneath the nest using their hind feet to push out dirt. June 13, This p.m. I dropped in to see the mice and found them under the nest in a cleared place they had dug out. All the areas on their body where the fur is white have now been well covered with fur. This fur came in later and slower than the dark dorsal pelage. June 14, On arriving this a.m. I found Pat and Mike had excavated a large space beneath the nest cavity. They seem now to prefer the cavity they have dug to the other nest I made for them. When I had removed them from the box for a few minutes to weigh them, I returned them by placing them in the man made nest. One lingered long enough to go inside and come out again; the other immediately headed for the burrow underneath. I then removed the nest and placed it in another corner of the box. They at once began to excavate beneath it in its new location. On June 1, when they were about 15 days old one of the mice while being held in my hand cracked the shell of a hard seed and peeled it off by