The Tropical Barrier is formed mainly by a climatic complex (probably a change in temperature and rainfall) in the coastal region at or about the latitude of Soto la Marina, where no geographic barrier is found. In the western and central part of the Tropical Barrier, the climatic factor is supported by a geographic factor. The Sierra Madre Oriental is in the west and the Sierra de Tamaulipas is in the center. The several mammals that are affected by this barrier are listed in the accounts of the Veracruzian and Tamaulipan biotic provinces. A peculiar pattern of distribution is that presented by _Scalopus inflatus_ and _Geomys tropicalis_. Both are the only known species of their genera in northeastern México. Each is isolated from other species of its genus. The nearest known record of _Scalopus_ is 45 miles northward and the nearest record of _Geomys_ is approximately 165 miles northward. A possible explanation for the distribution of these two kinds is that each was widely distributed in one of the glacial periods and when the glacier receded to the north these animals remained in Tamaulipas, where they evolved and formed distinct species. The two species, _G. tropicalis_ and _S. inflatus_, are fossorial and for this reason probably were able to resist inhospitable climates better than non-burrowing species. HISTORY OF MAMMALOGY In Tamaulipas the first exploration directed in substantial measure toward finding out about the mammalian fauna, at least as far as I know, was made by Dr. L. Berlandier, who traveled mainly in the northern half of the state. His collections provided specimens of several previously unknown mammals, which were described by Baird (1858). The original manuscript of Berlandier never has been published. About 1880 Dr. E. Palmer collected mammals in the southern part of Tamaulipas, in the area around Tampico. The results of his exploration were reported by J. A. Allen (1881). E. W. Nelson and E. A. Goldman twice collected in Tamaulipas (Goldman, 1951). In 1898 they visited and collected mammals in the southern part of the state, around Tampico, Altamira, Victoria, Forlón, and Miquihuana. In 1901-1902 they visited the area between Nuevo Laredo and Bagdad, then went south to Soto la Marina and Victoria. From their collections several species and subspecies have been described. Between 1910 and the early 1920's little was done in the way of scientific exploration because of the Mexican Revolution.