Study of the taxonomic resemblance shows that the dividing line, in eastern México, between Nearctic and Neotropical faunas is along the eastern base of the Sierra Madre Oriental, the southern base of the Sierra de Tamaulipas and thence to the coast at or near Soto la Marina. PLANT-MAMMAL RELATIONSHIPS Merriam (1898) assigned to Tamaulipas four Life-zones. There were: Transitional on the highest elevations of the Sierra Madre; Upper Austral at lower elevations on the Sierra Madre; Lower Austral over most of the state; and Tropical in the coastal areas. Dice (1943) outlined Biotic Provinces on a map of North America and in the northern part of Tamaulipas showed two Biotic Provinces, Tamaulipan and Potosian. He did not show the southeastern limits of the Chihuahuan Biotic Province nor any of the limits of the Veracruzian Biotic Province and in text mentioned nothing about the limits of these two provinces with reference to Tamaulipas. Later, Goldman and Moore (1946) divided Tamaulipas in three Biotic Provinces: Tamaulipas, Sierra Madre, and Veracruz. Still later (1949), Smith published a map of Mexican Biotic Provinces based on the herpetofauna of the Republic. He divided Tamaulipas among four Provinces. Two were Nearctic (Austro-oriental and Tamaulipan) and the other two were Neotropical (Veracruzian and Cordoban). Leopold (1950 and 1959) recognized five principal vegetational types in Tamaulipas as follows: Mesquite-grassland; Pine-oak Forest; Thorn Forest; Tropical Deciduous Forest; and Desert. For dealing with the mammals of Tamaulipas in the following accounts the four Biotic Provinces (Tamaulipan, Potosian, Veracruzian, and Chihuahuan) of Dice are the most useful. For dealing with types of vegetation in the accounts that follow, Leopold's (1950) system is employed although reference is made to other associations and formations that have been reported in Tamaulipas. Tamaulipan Biotic Province This Province is recognized by most authors who have written about the zoogeography of México. It is the most extensive in the state and includes the northern part of the Coastal Plain (see Fig. 2). The vegetation of the Tamaulipan Biotic Province is in general Mesquite-grassland but in the Sierra San Carlos and Sierra de Tamaulipas other types of vegetation are found. Two formations occur in the Mesquite-grassland. The first is the Mesquite Scrub, in which the dominant plant is the mesquite (_Prosopis juliflora_), associated with _Cordia boissieri_, several species of _Acacia_, and in some areas with _Opuntia_ and _Yucca treculeana_. The dominant grasses are of the genera _Bouteloua_ and _Andropogon_. The second formation is the Gulf Bluestem Prairie, where species of _Andropogon_ are the dominants on the well-drained sites. Sloughs and depressions are occupied by cordgrass, _Spartina spartinae_. Many areas have been invaded by mesquite and other shrubs.