_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. PHYLETIC TRENDS 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 prooedont 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 prooedont 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.