Diagnosis.—Ramus small for the genus; molar teeth depressed; cingular ridges (ectolophids) of m1 and m2 well-developed; anterior median fold present (appearing larger owing to chip of enamel missing); external reëntrant fold of m3 progresses half way across crown of tooth; diastema short; incisor moderately large, recurved; length of molar row, 2.6 mm.
Comparisons.—For comparisons with B. brachygnathus, B. kolbi, and B. sawrockensis, see accounts of those species. From B. rexroadi, B. minimus [Pg 592] differs in: anterior median fold deeper; incisor longer, more recurved, less proödont; molars slightly more depressed (though worn).
From B. musculus, B. minimus differs in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed.
From B. taylori, B. minimus differs in: anterior median fold slightly deeper; molar teeth more depressed; cingular ridges on m1 and m2 better developed; masseteric ridge more depressed.
Remarks.—Gidley (1922:124) stated that B. minimus differed considerably from B. taylori in that the coronoid portion of the ascending ramus diverges at a wider angle from the alveolar part of the jaw. Study of large samples of lower jaws of B. taylori reveals considerable individual variation in the angle formed between the coronoid part of the jaw and the alveolar part.
B. minimus, except for its small size, is like B. musculus and is considered to be ancestral to that species.
It seems that the important trends in phyletic development in the pygmy mice have been from an ancestral stock (see [Figure 3]) that possessed relatively brachydont teeth having raised cingular ridges (ectolophids and mesolophids) and relatively short orthodont to proödont incisors, to species having teeth more hypsodont on which cingular ridges were reduced, stylids were isolated or completely absent, and incisors were longer and more recurved or retrodont. Baiomys sawrockensis, or an unknown stock resembling it, might have been ancestral to the other known species. Of the four remaining fossil species, B. kolbi seems least likely to have been ancestral to the two living species, owing to its proödont incisors, reduction of cingular ridges, loss of an anterior median fold in m1, and long mandibular tooth-row. B. kolbi may have been an early, specialized derivation from the ancestral stock. From his knowledge of the habitats of B. musculus, the larger species, and B. taylori, the smaller species, Hibbard (1952:203) suggests that B. kolbi, a large species, might have inhabited lowlands, and B. rexroadi, a small species, highlands. I have no evidence to dispute this suggestion except that B. musculus has more prominent cingular ridges (or at least vestiges of this lophid condition) than either B. kolbi or B. rexroadi. B. musculus (see [page 610]) is less of an open grassland inhabitant than is B. taylori. Therefore, both B. kolbi and B. rexroadi, because of their poorly developed cingular ridges, might be expected to have lived in a relatively open grassland habitat.
The relationship of B. rexroadi to fossil species other than B. kolbi is not clear. Superficially, the former resembles B. taylori, but, owing to the specialized development of the molars of rexroadi, it could hardly have been ancestral to either of the living species. The resemblance of B. rexroadi to B. taylori may result from each having occupied the same ecological niche in different periods. The incisors of B. rexroadi, however, are much shorter than those of B. taylori and suggest somewhat different food habits.