In the 3520 specimens examined, an extra tooth was observed in only one (see Hooper, 1955:298). The left mandibular tooth-row of an adult male (USNM 71539) from Omentepec, Guerrero, is worn more than the right one. Irregularities in number of teeth and abnormalities in individual teeth seem to be rare in pygmy mice.
Table 3.—Individual Variation: Coefficients of Variation for Dimensions of External
and Cranial Parts in a Population of B. Musculus and B. Taylori.
| Measurement | Baiomys taylori | Baiomys musculus | ||
| Vic. (see [page 595]) Altamira, Tamaulipas | Vic. (see [page 595]) El Salvador | |||
| 21 Males C. V. | 18 Females C. V. | 17 Males C. V. | 13 Females C. V. | |
| Total length | 3.0 | 4.7 | 4.9 | 3.7 |
| Length of tail | 5.7 | 5.9 | 6.2 | 6.4 |
| Length of body | 2.8 | 5.0 | 5.9 | 3.5 |
| Length of hind foot | 3.7 | 3.4 | 3.0 | 4.1 |
| Length of ear | 0.0 | 0.0 | 5.5 | 3.3 |
| Occipitonasal length | 2.2 | 2.7 | 1.8 | 2.3 |
| Zygomatic breadth | 3.6 | 3.3 | 2.2 | 2.7 |
| Interorbital breadth | 3.2 | 3.3 | 2.2 | 2.9 |
| Incisive foramina (length) | 3.8 | 4.6 | 2.5 | 3.2 |
| Depth of cranium | 3.6 | 2.5 | 2.5 | 2.5 |
| Alveolar length, upper molars | 2.7 | 2.5 | 2.8 | 3.2 |
| Postpalatal length | 3.1 | 4.7 | 2.1 | 2.9 |
| Length of rostrum | 3.3 | 3.6 | 2.4 | 4.7 |
| Breadth of braincase | 2.7 | 1.4 | 4.0 | 4.9 |
The posterior margin of the bony palate varies from semicircular to nearly V-shaped. The suture between the nasals and frontals varies from V-shaped to truncate to W-shaped. The maxillary part of the zygoma varies from broad to slender in dorsoventral width in both species.
There are three distinct pelages, juvenal, postjuvenal, and adult. The sequences of molt and change of pelage from the juvenal, to the postjuvenal, and from it to adult, are essentially as reported for Peromyscus by Collins (1918:78-81; 1924:58-60) and Hoffmeister (1951:5). The juvenal pelage is uniformly dusky gray throughout except for the paler gray on the venter. In most juvenal mice, the yellow to ochraceous pigments of the subterminal bands are reduced or absent. Unlike Peromyscus, Baiomys has bright brownish hairs on the head as the first evidence of the postjuvenal molt (see [Figure 4, part a]). Blair (1941:381) reports adult pelage in pygmy mice being evident first at an age of 46 days. Two of my juveniles born in captivity began the postjuvenal molt on the 38th and 40th days. The area of new hairs on the head spreads most rapidly posteriorly. New hair appears ventrally and laterally at the end of 46 days (see [Figure 4, part b]). Hair replacement proceeds more slowly after the "saddle back" stage (described in Peromyscus by Collins, 1918:80) has been reached. That stage was reached in two pygmy mice at 52 days (see [Figure 4, part c]). Areas immediately posterior to the ears, in the scapular region, molt last. The postjuvenal pelage was seemingly complete in one captive pygmy mouse at the end of 60 days. Another captive failed to complete its growth of new pelage until two additional weeks had elapsed. Length of time required to molt in pygmy mice is about the same as that reported by Layne (1959:72) in Reithrodontomys.
Fig. 4. Diagrams showing progress of the postjuvenal molt in pygmy mice.
For explanation of a, b, and c, see text. All approximately 2/3 natural size.