Because of the wide range of variation in some of its characters, _P. b. ambiguus_ is difficult to diagnose. Nevertheless, its small external and cranial size, short anterior palatine foramina, and bright color seem to separate it from other subspecies of _P. boylii_ in the eastern part of the range of the species. These differences are most conspicuous when specimens from the northernmost part of the range of _levipes_ are compared with specimens of _ambiguus_. The specimens from the Sierra San Carlos, Tamaulipas, closely resemble _levipes_ in color, but are referred to _ambiguus_ on the basis of small size, as also are the two specimens from 12 km. E San Antonio de las Alazanas, Coahuila. TABLE 1. MEASUREMENTS (IN MILLIMETERS) OF PEROMYSCUS A: Number of specimens B: Total length C: Length of tail-vertebrae D: Length of hind foot E: Per cent length of tail to head and body F: Greatest length of skull G: Zygomatic breadth H: Interorbital constriction I: Length of nasals J: Palatine slits K: Maxillary tooth-row =======+======+======+=====+======+=====+=====+====+=====+====+==== A | B | C | D | E | F | G | H | I | J | K -------+------+------+-----+------+-----+-----+----+-----+----+---- _P. aztecus_ Mirador, Veracruz -------+------+------+-----+------+-----+-----+----+-----+----+---- 7 mean| 229 | 113 | 24.5| ... | 30.1| 15.3| 4.7| 12.5| 6.4| 4.8 max.| 238 | 121 | 26 | ... | 30.9| 15.8| 5.0| 13.5| 6.8| 5.0 min.| 215 | 107 | 24 | ... | 29.2| 14.9| 4.6| 11.2| 5.7| 4.7 -------+------+------+-----+------+-----+-----+----+-----+----+---- _P. boylii beatae_ Las Vigas to 3 km. E thereof, Veracruz -------+------+------+-----+------+-----+-----+----+-----+----+---- 14 mean| 219.3| 116.7| 23.8| 113.7| 28.9| 14.4| 4.5| 11.5| 6.3| 4.5 max.| 235 | 130 | 25 | 128.9| 29.8| 15.1| 4.7| 12.5| 6.8| 4.8 min.| 204 | 107 | 22 | 100.0| 27.9| 13.8| 4.2| 10.7| 5.9| 4.4 -------+------+------+-----+------+-----+-----+----+-----+----+---- 6 km. SSE Altotonga, Veracruz -------+------+------+-----+------+-----+-----+----+-----+----+---- 5 mean| 224.4| 116.1| 24.1| 109.4| 29.1| 14.5| 4.5| 11.6| 6.4| 4.5 max.| 241 | 126 | 25 | 114.8| 30.1| 15.2| 4.6| 12.0| 6.7| 4.7 min.| 221 | 110 | 24 | 100.0| 28.6| 14.0| 4.4| 11.2| 6.0| 4.3 -------+------+------+-----+------+-----+-----+----+-----+----+---- _P. boylii levipes_ 3 km. SW San Isidro, San Luis Potosí -------+------+------+-----+------+-----+-----+----+-----+----+---- 11 mean| 205.6| 99.5| 22.4| 93.8| 28.5| 14.2| 4.4| 11.3| 5.9| 4.4 max.| 219 | 114 | 23 | 108.6| 30.5| 14.5| 4.6| 12.2| 6.4| 4.6 min.| 193 | 90 | 21 | 87.4| 27.2| 13.8| 4.2| 10.8| 5.6| 4.1 -------+------+------+-----+------+-----+-----+----+-----+----+---- 6 km. NE Cd. del Maíz, San Luis Potosí