The route that Baker (1956:146) termed the "Southern Route" I here term the Montane route because I think it was used for movement southward as well as northward. [Illustration: FIG. 3. Routes of movement: 1 Northern; 2 Trans-Plateau; 3 Montane; 4 Tropical.] The Montane route was used by mammals of boreal affinities (_Microtus_ and _Neotoma_), that moved into Tamaulipas from the north; also in this category are bats of the family Vespertilionidae. For movement from south to north, the route was used by several species native to México, for example, _Cratogeomys castanops_. The seaward slope of the montane area has enabled some tropical mammals to move farther north than they have done at higher and lower elevations. _Philander opossum_ seems to be an example. The fourth route, the Tropical one, was used by mammals of tropical origin. Most moved into Tamaulipas only as far as the Veracruzian Biotic Province. The principal mammals that have used this route are the bats and marsupials, but _Sylvilagus brasiliensis_, _Ateles geoffroyi_, _Heterogeomys hispidus_, _Eira barbara_, and _Mazama americana_ also can be included here. Some tropical mammals, as was pointed out previously, not only reached Tamaulipas but have moved through the state and far northward. The major barriers to dispersal of mammals in Tamaulipas are three (see Fig. 2). Two of them, the Río Grande Barrier and the Sierra Madre Barrier, are physiographical, but the Tropical Barrier is maintained by a combination of environmental factors. The three barriers separate the four Biotic Provinces in Tamaulipas. The Sierra Madre Oriental, which forms the Potosian Biotic Province, lies between the Tamaulipan and Chihuahuan provinces. The Tropical barrier separates the Tamaulipan and Veracruzian biotic provinces. The Río Grande, as was pointed out by R. H. Baker (1956:146), has low banks, is relatively shallow, and does not form an effective barrier for most mammals. For only two species, insofar as I know, has the Río Grande constituted a barrier. _Cratogeomys castanops_ has not entered southeastern Texas from México, and _Spermophilus spilosoma_ has not entered México from southeastern Texas except on the coastal barrier beach. Alvarez (1962:124) postulated that the beach was the route by which _S. spilosoma_ arrived at La Pesca where the barrier beach meets the mainland. The Sierra Madre Barrier is a good filter for some small mammals, especially for those that occur on the Mexican Plateau and those of tropical origin. The mammals that occur on each side of the Sierra are listed in accounts of the Chihuahuan (west side), Veracruzian and Tamaulipan (east side) biotic provinces.