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, ♂ 5.7 (5.0-7.0) versus 4.5, and in ♀ 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.
Figs. 1-3. Three views of the skull of the type specimen of Thomomys umbrinus pullus. × 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 Michoacán. 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 Pátzcuaro, have the authors found two kinds of pocket gophers living together. The two-story arrangement south of Pátzcuaro 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.
Measurements.—Average and extreme measurements of five adults of each sex, are as follows: Total length, male 184 (178-198), female 185 (174-194); length of tail, 54 (48-60), 53 (47-57); length of hind foot, 26.8 (25-29), 27.6 (26-29); weight, 86.1 (78.7-96.9), 74.3 (70.2-84.8) grams; basilar length, 30.2 (28.8-31.3), 28.6 (27.8-29.1); zygomatic breadth, 23.2 (22.3-24.6), 21.3 (20.8-21.8); least interorbital breadth, 5.9 (5.8-6.1), 6.4 (6.0-6.8); mastoid breadth, 17.8 (17.1-18.7), 17.2 (16.6-17.5); length of nasals, 12.4 (11.8-13.0), 11.5 (11.0-12.5); breadth of rostrum, 7.5 (6.9-8.2), 7.1 (6.9-7.3); length of rostrum, 14.1 (13.4-14.5), 13.3 (12.7-13.5); alveolar length of maxillary tooth-row, 7.0 (6.7-7.5), 6.9 (6.8-7.0); palato-frontal depth, 13.2 (13.0-13.4), 12.9 (12.3-13.5).
Specimens examined.—Total, 17, all from 7800 ft., Michoacán, as follows: 3 mi. S Pátzcuaro, 1; 4 mi. S Pátzcuaro, 10; 5 mi. S Pátzcuaro, 6.