E.V. Miller 1940 Perognathus penicillatus March 19 2450' + Cooper's Well, 47 mi. S. Marathon, Brewster Co., Texas. Took 5 of in mouse traps set on sandy plain with medium thick growth of creosote and mesquite brush. Also took Peromyscus eremicus, Dipodomys ordii and D. mericanus, in same habitat. May 23 Potrero Santa Ana, mi S Matehuala, San Luis Potosi. Took 5 of this species in traps set at the edge of a field lined with thick mesquite brush. Soil a little sandy, light reddish brown color, very muddy at present from a recent rain. May 25 Lulu, Zacatecas Took one male of this species in a trap set near weeds on sandy soil. May 27 3 mi. N Lulu, Zacatecas. Took one male. In the evening as M. Delgadillo and I were sitting in the tent, at about 9:30 P.M. an immature P. penicillatus appeared inside the tent, about 5 ft. from us, foraging busily and not appearing to be alarmed at the gas lantern. The mouse moved about almost continually, in short runs of 6 inches to a foot, hardly stopping a second to pick up food. It remained only a minute or so before departing into the night.