Representatives of nine species of _Dipodomys_ were dissected in an
attempt to determine the degree of specialization and the relative
systematic position of each species.
The myology was found to agree in detail as to origin, insertion and
innervation with that of _Dipodomys spectabilis_ as reported by Howell
(1932). The only variation noted in the muscular system was the size of
the individual muscles in those animals of widely divergent body size.
_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.