_Remarks._--_Peromyscus ochraventer_ is considered to be a distinct species showing little evident relationship with other Mexican _Peromyscus_. In the shape of the skull, especially the anterior expansion of the rostrum, _P. ochraventer_ seems to be related to _P. furvus_ and _P. latirostris_, a series of the latter being made available for examination by Dr. George G. Lowery, Jr., of the Museum of Zoology at Louisiana State University. However, the rostrum of these two larger species is proportionately longer than the rostrum of _P. ochraventer_. In size, coloration and most cranial features, _P. ochraventer_ resembles _P. mexicanus_, although the absence, instead of presence, of a supraorbital bead or ridge, the almost parallel-sided, instead of more pointed, rostrum and the larger, instead of smaller, interiormost loph of the first upper molar in _P. ochraventer_ are well-marked differences. The baculum of _P. ochraventer_ is much shorter with a proportionately heavier base and shaft than the baculum of _P. mexicanus mexicanus_ (from Veracruz) and _P. m. saxatilis_ (from Costa Rica). The geographic range of _Peromyscus ochraventer_ is not known to meet that of _P. mexicanus_; the nearest place to the type locality of _P. ochraventer_ from which _P. mexicanus_ has been taken is at Xilitla approximately 225 kilometers to the southward in San Luis Potosi (Dalquest, Occ. Papers Mus. Zool., Louisiana State Univ., No. 28:8, July 10, 1950). The brown coloring on the underparts is a distinctive feature of _P. ochraventer_; in adults this color differs in shade. In some specimens patches of whitish hair give the tail a splotched appearance. Eleven of the twenty-eight skulls and lower jaws examined have bone eroded away from around the cheek-teeth exposing part of the roots. Most of the fully adult animals have this condition. One adult female, no. 36959, has the upper third molar on the right side missing, possibly as a result of bone erosion. These mice were taken in junglelike forest, in rocks and adjacent to logs. Schaldach writes that "Their burrows go back under the large limestone blocks, and each burrow where I caught a mouse has a pile of excavated earth, like a tiny gopher mound." The trapping area was at an elevation of approximately 2800 feet on the steep sides of a small hill on top of which the field camp was situated. Schaldach indicated that this locality was transitional between arid tropical and humid tropical conditions. On January 13, 1950, a female taken was lactating and had five recent placental scars; another taken the same day also had five placental scars.