The intermontane complex consisting of _D. o. montanus_, _longipes_, _cupidineus_, _nexilis_, _sanrafaeli_ and _uintensis_, like the western desert complex, has become separated from the remainder of the subspecies of the species _D. ordii_ by Quaternary geologic events. _D. o. cupidineus_ has been cut off by the gorges of the Colorado River to the south and the Virgin River to the north. _D. o. sanrafaeli_ is separated from _D. o. uintensis_ by the Tavaputs Plateau and by the Roan and Book cliffs, and is separated from the range of _D. o. nexilis_ by the Colorado River although there is intergradation between _D. o. nexilis_ and _D. o. sanrafaeli_. _D. o. longipes_ has been separated from the rest of this intermontane complex by the San Juan and Colorado rivers, but to the east it intergrades freely with adjacent subspecies. _D. o. montanus_ has been relatively isolated in the San Luis Valley of Colorado and New Mexico, but in the southern part of its range it does show intergradation with other subspecies. [Illustration: FIG. 22. An arrangement, according to morphological indices, of the subspecies of _Dipodomys ordii_.] The complexes mentioned above are represented graphically in Figure 22, in a way that expresses some of my ideas as to their genetic relationships. The indices used to determine the amount of specialization that each complex of subspecies has undergone are as follows: The Body index (head and body/length of tail × 100) is the expression of the elongation of the tail as an organ of balance while the length of the head and body remain relatively constant. As the tail elongates the index decreases and as the tail becomes shorter the index increases. The Pedal index (hind foot/head and body × 100) is the expression of the development of the hind foot as an element essential for the saltatorial habit. As the hind foot elongates the index will increase; elongation of the hind foot is interpreted as a specialization. The Cranial index (breadth across bullae/length of the skull × 100) reflects the degree of development of the tympanic or mastoid region, or both, and is thought to be an adaptation for more acute hearing and possibly for more delicate balance. Inflation of the tympanic bullae is thought to be a specialization. As the auditory bullae become more inflated, the index increases toward 100. The Bullar index (width of maxillary arches/breadth across bullae × 100) also expresses the degree of inflation of the auditory bullae. In a generalized mammal, at least in the heteromyids, the index would be 100, but as the auditory bullae become larger the index will decrease from 100.