The skeleton, particularly of the appendages, shows the most modification, ranging from a relatively generalized to a specialized condition. Skeletal indices, as established by Howell (1944:199) have been used in estimating the amount of such specialization. These indices are obtained by dividing the length of one segment of a limb by the length of another segment and are expressed in percentages. The Femorotarsalmetatarsal and Cranial indices are not from Howell (_loc. cit._). The Humeroradial index (radius/humerus ×100) in the generalized animal is theoretically 100 because the humerus and radius are of the same length. In kangaroo rats, which are saltators, the index rises to more than 100 owing to the lengthening of the radial component. The Intermembral index (humerus and radius/femur and tibia ×100) in a generalized animal is theoretically 100, but, as Howell (1944:205) points out, the index in generalized mammals is probably nearer 75. If the hind leg elongates at the expense of the forelimb the animal will be a better saltator and the skeletal elements will yield a lower intermembral index. The Femorotibial or Crural index (tibia/femur ×100) expresses the development of the tibia as an adaptation to the saltatorial habit and in generalized animals would be expected to be 100. As an adaptation to saltation the tibia would elongate at the expense of the femur and the index would be more than 100. The degree of divergence from 100 would be an expression of the degree of saltatorial ability. The Tibioradial index (radius/tibia ×100) in the generalized animal also would be expected to approximate 100 but it is doubtful if any living mammals, except brachiating kinds, yield an index of more than 75. In saltators, the index is low because of the elongation of the hind appendages, whereas the forelimbs do not change their length or are shortened. The Femorotarsalmetatarsal index (tarsometatarus/femur ×100) in the generalized condition would be less than 50 and an index approaching 100 would indicate a specialization for saltation owing to the elongation of the tarsometatarsal elements. The Cranial index (breadth across bullae/length of skull ×100) reflects the development of the auditory or mastoid region of the skull as an adaptation for more acute hearing and possibly for more delicate balance. In heteromyids, the generalized condition would be represented by an index of 50 or less, and as the width across the bullae increases, the index rises toward 100. [Illustration FIGS. 2-10. Showing the compacting of the visceral mass; liver at the top, small intestine and caecum at the bottom. All figures approximately × 1. FIG. 2. _Dipodomys ordii inaquosus_, [M], adult, no. 23365, KU; 7 mi. W Fallon, Churchill County, Nevada; trapped 27 October, 1945. FIG. 3. _Dipodomys panamintinus mohavensis_, [M], adult, no. 22094, KU; 1-1/2 mi. N Mojave, Kern County, California; 3 February, 1948.