The dens of the two larger species of _Dipodomys_--_spectabilis_ and _deserti_--can be distinguished at a glance from those of the two smaller--_merriami_ and _ordii_--by the fact that the mounds of the former are usually of considerable size and the burrow mouths are of greater diameter. On the Range Reserve _merriami_ erects no mounds, but excavates its burrows in the open or at the base of _Prosopis_, _Lycium_, or other brush. The mounds of _spectabilis_ are higher than those of _deserti_, the entrances are larger, and they are located in harder soil (Pl. III, Fig. 1). The dens of _deserti_ are usually more extensive in surface area than those of _spectabilis_, and have a greater number of openings (Pl. III, Fig. 2). [Illustration: PLATE II. FIG. 1.--WINTER VIEW OF AREA INHABITED BY KANGAROO RATS. A water-hole scene on the U. S. Range Reserve at the base of the Santa Rita Mountains, Ariz., where cooperative investigations are being conducted to ascertain the relation of rodents to forage. This is typical of a large section of country occupied by _Dipodomys spectabilis spectabilis_ and _Dipodomys merriami_. The brush is mesquite (_Prosopis_), cat's-claw (_Acacia_), and paloverde (_Cercidium_).] [Illustration: PLATE II. FIG. 2.--KANGAROO RAT COUNTRY FOLLOWING SUMMER DROUGHT. An area of the U. S. Range Reserve in the autumn of 1918, showing the result of failure of summer rains. Such a condition is critical not only for the stockmen but also for kangaroo rats and other desert rodents, and results in competition between them as to which shall benefit by what the range has to offer.] [Illustration: PLATE III. FIG. 1.--KANGAROO RAT MOUND (DIPODOMYS S. SPECTABILIS). Typical _Dipodomys s. spectabilis_ mound on the Range Reserve, under shelter of desert hackberry (_Celtis pallida_). Most dens on the reserve are located in the shelter of brush plants, the more important being mesquite (_Prosopis velutina_), cat's-claw (_Acacia_ spp.), and the desert hackberry. (See also Pl. VIII Fig. 2.)] [Illustration: PLATE III. FIG. 2.--KANGAROO RAT MOUND (DIPODOMYS DESERTI). Den of _Dipodomys deserti deserti_, showing typical wide, low mound with numerous entrance holes. This species excavates its den in soft, sandy soil. The tree is a species of _Dalea_.] DESCRIPTION. GENERAL CHARACTERS. Size large; ears moderate, ear from crown (taken in dry skin) 9 or 10 millimeters; eyes prominent; whiskers long and sensitive; fore feet short and weak; hind feet long and powerful, provided with four well-developed toes; tail very long, usually 30 to 40 per cent longer than the body. Cranium triangular, the occiput forming the base and the point of the nose the apex of the triangle, much flattened, auditory and particularly mastoid bullae conspicuously inflated. COLOR.