_Comparison._--From topotypes of _Thomomys umbrinus supernus_ Nelson and Goldman, _pullus_ differs as follows: More individuals wholly black (except distal half of tail); underparts lacking white; rostrum broader; braincase anteriorly slightly more expanded dorsally; lambdoidal crests perpendicular to sagittal plane rather than inclined posteromediad; interparietal broader, [Male] 5.7 (5.0-7.0) versus 4.5, and in [Female] 6.5 (5.6-7.1) rather than 4.8 (4.4-5.1); flattened middle part of jugal vertical rather than oblique; in side view, mastoid and paroccipital processes farther apart thus exposing larger surface of mastoidal bulla; incisors, in both upper and lower jaws, slightly narrower; molariform teeth smaller, interpterygoid space truncate, at apex, with sides convex mediad, rather than V-shaped; ventral face of tympanic bullae rugose in posterior half rather than smooth. [Illustration: FIGS. 1-3. Three views of the skull of the type specimen of _Thomomys umbrinus pullus_. x 1.] _Remarks._--Among named subspecies of _Thomomys umbrinus_, _T. u. pullus_ most closely resembles _T. u. supernus_, the subspecies next adjacent to the northward. Therefore, the results of comparisons with only that subspecies are here reported upon. _T. u. tolucae_ to the eastward is for one thing a much larger animal and has slightly less procumbent upper incisors. So far as we know, _Thomomys umbrinus_ has not heretofore been reported from Michoacan. Of our seventeen skins, eight are brown, six are black and two are intermediate in color. Most of these pocket gophers lived where there was a good growth of pine trees in the same areas where large pocket gophers of the species _Cratogeomys gymnurus_ occurred. The field notes of the collector of the type of _T. u. pullus_ record that when he was making a shallow excavation to reveal the gopher burrow in which he trapped the holotype, he found the burrow approximately five inches below the surface of the ground and that in digging deeper than was necessary he accidentally broke into the burrow of a _Cratogeomys_. Another member of our field party (E. R. Hall) when removing from its burrow a trapped _Thomomys_ that was caught only by the hind leg, dug around the animal whose burrow was approximately six inches underground and in doing so he also broke through the roof of a burrow of _Cratogeomys_. The burrow of _Cratogeomys_ was approximately sixteen inches below the ground. Nowhere else, except 3 to 5 miles south of Patzcuaro, have the authors found two kinds of pocket gophers living together. The two-story arrangement south of Patzcuaro was possible because of the different levels at which the two kinds of animals made their burrows and the two-story arrangement was accidental and exceptional rather than the rule.