Quart 1948 Perognathus arenarius 3 June 19 San Jorge, 5+ ft, 25°44'N, 112°07'W. Baja California. few Ocotillo and an occasional Lumbi. The character of the sand is such that it packs firmly, being fine and of a reddish tint. The Museum special was baited with walnut. One Perognathus baileyi was also caught in this habitat. 48 live traps set among Ocotillo, Lumbi, and Pitiaya agria clumps on hill east of camp last night caught 9♂ + 8♀ of this species and also 2 Dipodomys merriami + one Perognathus baileyi (#388-405 incl). This hill is about 100 yards east of the estero shore and probably about 75 feet high being com- posed of reddish hard-pack sand containing a few shells (probably of human origin). Mouse sign is abundant in this habitat and small burrows exceedingly common. The moon was almost full last night. June 20 Same location 50 live traps set in same habitat but N.F. of camp caught 4♂ and 3♀. The females had no embryos, 1 Dipodomys merriami and one Perognathus baileyi also caught in same habitat. June 21 Same location Moved my live traps up to the north end of the estero last night. At that place a Mangrove thicket exists that extends about 100 yards north from the estero in boggy ground and is peculiar in its fact that it is made