[Illustration: FIG. 25. Known occurrences and probable geographic range of the subspecies of _Dipodomys ordii_ in the northeastern fourth of the range of the species. 1. _D. o. richardsoni_ 8. _D. o. terrosus_ 18. _D. o. montanus_ 2. _D. o. oklahomae_ 10. _D. o. uintensis_ 24. _D. o. priscus_ 5. _D. o. evexus_ 11. _D. o. sanrafaeli_ 31. _D. o. longipes_ 6. _D. o. medius_ 15. _D. o. luteolus_ 33. _D. o. nexilis_ ] From _Dipodomys ordii obscurus_, _D. o. extractus_ differs as follows: Size larger; color lighter; skull larger in all respects, notably in least interorbital width, greatest length of skull and breadth across auditory bullae; zygomatic arches less robust and straight as opposed to robust and bowed laterally; pterygoid fossae subcircular as opposed to circular; auditory bullae more inflated ventrally. From _Dipodomys ordii idoneus_, _D. o. extractus_ differs in: Size larger; color lighter; skull larger in all measurements taken except least interorbital width and breadth across maxillary arches which are smaller; nasals longer and narrower; braincase less vaulted; zygomatic arches less robust and more nearly straight; auditory bullae less inflated ventrally. _Remarks._--This subspecies of _Dipodomys ordii_ is the palest of any of the known races from Mexico. It inhabits an area of light-colored sandy soil at the type locality. The only subspecies with which _D. o. extractus_ really needs comparison is _Dipodomys ordii ordii_ from which it differs as described above. The series which is herein described contains two individuals that resemble _D. o. ordii_ cranially and three that resemble it in color although they are somewhat paler. One specimen, MVZ no. 76560, is the palest of any of the series, has a short tail and in these respects resembles the pale form of _Dipodomys ordii compactus_. It is apparent from the skull of this latter individual that it is not _D. o. compactus_ but _D. o. extractus_. _Dipodomys ordii ordii_ inhabits, in most of its range, dark-colored soils, whereas _D. o. extractus_ inhabits light-colored soils. Possibly the differentiation that has occurred is a result of an ecological separation much as occurs with _Perognathus_ on the white sands and black lavas of the Tularosa Basin of New Mexico (see Benson, 1933). _Specimens examined._--Total, 14, all from Chihuahua, as follows: 1 mi. E Samalayuca, 4500 ft., 14 (MVZ). =Dipodomys ordii chapmani= Mearns _Dipodomys chapmani_ Mearns, Bull. Amer. Mus. Nat. Hist., 2:291, February 21, 1890. _Cricetodipus chapmani_, Trouessart, Catalogus Mammalium, 1:581, 1897. _Perodipus chapmani_, Elliot, Field Columbian Museum, Zool. Ser., 2:237, 1901. _Perodipus ordii chapmani_, Goldman, Proc. Biol. Soc. Washington, 30:113, May 23, 1917. _Dipodomys ordii chapmani_, Grinnell, Journ. Mamm., 2:96, May 2, 1921.