_Dipodomys ordii_ is the most generalized and _Dipodomys deserti_ is the most specialized of the kangaroo rats (see Table 1), as judged by the osteology. Information gained by the study of the viscera of the various species supports this judgment. The visceral mass is relatively loose in _D. ordii_, but is markedly compact in _D. deserti_. This compactness appears to be brought about by the foreshortening of the mesenteries which support the entire gut and by the closer apposition of the large intestine to the caecum; both the intestine and caecum occupy a ventral position in the abdominal cavity. In _Dipodomys ordii_ the entire visceral mass is loosely interconnected and the caecum is relatively small as compared to the tightly compact viscera and the large caecum in _Dipodomys deserti_. Another striking feature is the size, proportion and position of the liver. In all animals dissected, even in _D. deserti_, the right lobe of the liver descends and forms a capsule around the anterior end of the right kidney. In the Ord kangaroo rat, the bulk of the liver lies on the right side of the body cavity. That is to say, there is a greater bulk of the liver on the right side and it is situated more dorsad than in any of the other species examined. In the most specialized condition, as in _Dipodomys deserti_, the bulk of the liver is almost equal on the right and left sides, and instead of having the greater bulk situated dorsally as in _D. ordii_ it is cup-shaped, with the dorsal and ventral parts of approximately equal size and situated on almost the same transverse plane. The entire mass of the liver is concave posteriorly. TABLE 1 SKELETAL INDICES OF DIPODOMYS ====================================================== Humeroradial | Intermembral | | Crural | | | Tibioradial | | | | Femorotarsal- | | | | | metatarsal | | | | | Cranial | | | | | | ------------------------------------------------------ ordii 144.5 57.2 127.75 60.55 88.4 63.4 microps 138.5 56.17 132.3 57.27 90.95 60.8 panamintinus 146.1 55.3 132.0 57.5 90.5 60.8 agilis 147.0 55.05 133.65 57.25 94.55 62.65 heermanni 142.9 54.2 135.9 55.35 92.2 60.93 ingens 142.9 54.1 130.6 56.2 89.65 66.2 spectabilis 140.9 53.05 133.9 54.2 95.6 64.6 phillipsii 163.4 55.05 137.85 58.97 101.5 64.5 merriami 160.75 53.85 137.5 57.35 99.75 63.9 nitratoides 155.0 54.1 137.4 57.0 98.25 65.5 deserti 149.5 53.4 139.4 54.9 96.6 67.6 ------------------------------------------------------