Perhaps the greatest amount of variation in the skull is in features which are not readily measurable by the usual physical means. The shape and size of the pterygoid fossae vary from almost round to rather ovoid in a given series of animals from one locality; the size and configuration of the zygomatic arches vary from slender to robust and from straight to curved laterally; the size of the lacrimal processes varies much in any given series, as do also the degree of expansion of the distal end of the nasals, the convexity of the braincase and the curvature of the upper incisors. In all instances where these features varied much, one size or shape was more pronounced in the series than any other size or shape. Thus, when comparisons were made, the size and certain shapes were the criteria used in assigning the animals under consideration to a given subspecies. Subspeciation in _Dipodomys ordii_ seems to have been influenced by water barriers. It is known (Grinnell, 1922:28) that kangaroo rats lack the ability to swim. Large stable rivers such as the Colorado, Snake and Columbia serve as effective barriers to further dispersal of kangaroo rats. Streams that freeze over in the winter months, however, are not efficient barriers. This is indicated by the "blending" of morphological characters of _D. o. nexilis_ and _D. o. sanrafaeli_ along the Green River which freezes over. Any mountain which has vegetational belts above the Transition Life-zone would serve as a barrier to the dispersal of these animals. The Uinta Mountains, lying in an east-west direction, are interposed between the ranges of _D. o. priscus_ and _D. o. uintensis_. The high Wasatch Mountains and their associated outliers, lying in a north-south direction in Utah, serve as an efficient barrier to the east-west movement of kangaroo rats and as a result, the subspecies east of the mountain mass are remarkably different from those to the west. TABLE 4 PROPORTIONATE DECREASE OF NASALS =========================================================== Width Length Least Nasals of of interorbital ------------ rostrum nasals orbital Interorbital width × 100 ----------------------------------------------------------- terrosus 4.1 14.75 13.5 91.6 luteolus 4.35 13.9 12.95 93.1 evexus 4.3 14.35 13.75 94.8 montanus 4.1 13.5 12.65 93.8 ordii 3.5 13.3 13.0 97.7 idoneus 3.7 13.2 13.75 103.5 palmeri 3.3 12.8 13.0 101.1 -----------------------------------------------------------