The tail is moderately well haired, but the hairs are too scanty to hide the coarse scales. In P. collinus the tail is longer than the head and body, but in P. monticularis the tail is shorter than the head and body. The ears are rather long, especially in P. collinus, and the legs are short. The short, broad feet have the normal number of digits. The upper incisor teeth are not grooved.
Females usually have six mammae but occasionally have only four.
They are nocturnal, hiding by day under rocks or in crevices and stealthily creeping between the boulders at night. Reports indicate that rock mice are omnivorous in diet. Females probably produced one litter annually, with two or three young. A female P. collinus collected in September had two embryos.