Some species inhabit the scattered growth of plants in sandy areas, but they are more generally characteristic of harder and more rock-strewn soil, rocky mesas, and foothill slopes. There a few species make burrows in open ground, sometimes with a single hole, but most of them make their nests under rocks, in crevices, or in burrows sheltered by such desert bushes as Covillea, Bursera, Olneya, Cercidium, and mesquites. In these shelters pocket mice make little mounds a few inches high and ten or fifteen inches across. The mounds have several entrances on different sides, one of which generally shows signs of recent use, although by day it is kept closed from within by loose earth. Each of the many-entranced dens is occupied by a single animal. Early in the morning, before the wind fills them with dust, tiny trails are to be seen leading from these doorways toward the nearest feeding grounds and all about their haunts. The spiny and the silky pocket mice, sharing much the same arid region, have the same food plants and are preyed upon by the same enemies. The food of these mice consists mainly of small seeds, including the wild morning glory, wild sunflowers, wild parsnips, and a multitude of others characteristic of the various areas they occupy. Pocket mice are strictly nocturnal or crepuscular in habits and appear by day only when disturbed. If the plugged entrance to a burrow is opened, however, it will probably be quickly stopped up again from within by the annoyed householder. The young, in litters of from two to eight, are born at irregular times according to the latitude and general weather conditions. In the south at least several litters appear to be born each year, the young being noted almost every month. When camping alone for a few days in the desert near San Ignacio, in the middle of the peninsula of Lower California, I had a unique opportunity to learn something of the peculiarities of the various pocket mice. Three species were abundantly represented, including both the silky and the spiny kinds. They quickly learned that good hunting could be found in and about the tents for the rice grains and other scattered food and promptly took advantage of it. As soon as approaching darkness began to render objects indistinct, from their burrows among the surrounding bushes they swarmed into camp and were busy throughout the night minutely searching the ground under the shelter tent for every particle of food. In order to see these interesting visitors to better advantage I placed a candle on a small box in the middle of the tent. Five or six individuals, representing three species, often came within the circle of light at the same time. At first all were shy and when I made any sudden movement would leap in every direction, like grasshoppers, and quickly vanish. The smallest of the species, a member of the silky group, was the shyest of all and remained timid and reserved.