When Baker (_op. cit._) described _Neotoma angustapalata_ on the basis of two specimens from El Carrizo, he assigned the species to the _N. mexicana_ group because of the deep anterointernal re-entrant angle of M1. The deep angle found in _N. mexicana_ differs markedly from the typical condition in either _N. micropus_ or _N. albigula_. Study of the cranial characters and bacula of specimens of _N. micropus_ and _N. angustapalata_ tends to corroborate the statement of Hooper (1953:10), who commented on the taxonomic relationships of _N. angustapalata_ as follows: "It should be pointed out that all characters considered ... the specimens [_angustapalata_] appear to be large, deeply pigmented examples of the species _N. micropus_ notwithstanding the deep anterior fold in M1. The presence of that deep fold is far from an absolute character in the _mexicanus_ [_sic_] group." My study of 48 crania of _N. micropus_ from Tamaulipas reveals that the depth of the re-entrant angle of M1 is extremely variable, from almost absent in some individuals to deep (as in _angustapalata_) in others. Four specimens, one (56958) from the Sierra de Tamaulipas and three (56960, 56965, 56966) from the vicinity of Altamira, have the re-entrant angle as deep as in the holotype and topotype of _angustapalata_. Comparison of the bacula of the holotype and one topotype of _angustapalata_ with 15 bacula of _N. micropus_ reveal that on the average the baculum of _angustapalata_ differs from that of _micropus_ in being longer, and narrower at the base (greatest length, 7.1, width at base, 3.4 mm., in the topotype). One specimen of _N. micropus littoralis_ from the vicinity of Altamira, however, has a baculum of the same shape as in _angustapalata_ (this same specimen is one of the three from there in which the re-entrant angle of the M1 is deep). The shape of the baculum among specimens of _micropus_ is highly variable and bacula of specimens from different localities frequently are slightly different (see Fig. 5). [Illustration: FIG. 5. Bacula of _Neotoma_. All × 4. A, _Neotoma angustipalata_ (topotype, 37062). B, _Neotoma micropus micropus_ (4 mi. SW Nuevo Laredo, 89147). C, _Neotoma micropus littoralis_ (Sierra de Tamaulipas, 2 mi. S, 10 mi. W Piedra, 56957). ]