[Illustration: FIG. 2. Four biotic provinces: 1 Tamaulipan; 2 Potosian; 3 Chihuahuan; 4 Veracruzian.] Around the Sierra de Tamaulipas and in the area between it and the Sierra San Carlos the vegetation is Thorn Forest (Tropical Thorn Forest of Martin _et al._, 1954), in which the dominant plants are _Acacia_, _Ichthyomethia_, _Ipomea_, _Prosopis_, and _Cassia_. Another type of vegetation in the Sierra de Tamaulipas is the Tropical Deciduous Forest at 300 to 700 meters elevation, the trees of which are 20 meters high with a canopy averaging eight meters high (Martin _et al._, _op. cit._). The common species of trees belong to the genera _Tabebuia_, _Ipomea_, _Bombax_, and _Conzattia_. Species of _Bursera_, _Acacia_, and _Cassia_ are less abundant. In the low canyons _Bursera_, _Ceiba_, and _Psidium_, draped with lianas and various epiphytes, can be found. The Pine-oak Formation grows above an elevation of 800 meters in the Sierra de Tamaulipas and is characterized by _Pinus cembroides_, _P. nelsonii_, _P. teocote_, and _Quercus arizonica_. Martin _et al._ (_op. cit._) recorded Montane Scrub from the dry areas, between elevations of 600 and 900 meters. That scrub is formed by huisaches (_Acacia farnesiana_) along with a few oaks and some trees of the Tropical Deciduous Forest. The vegetation of the Sierra San Carlos was studied by Dice (1937) and divided into three life belts, each with several associations. For more information about the plants of each association and their related mammals see the publication of the mentioned author. Endemic mammals of the Tamaulipan Biotic Province, in the part of it that is in Tamaulipas, are the following: _Scalopus inflatus_; _Lepus californicus curti_; _Spermophilus spilosoma oricolus_; _Cratogeomys castanops tamaulipensis_; _Dipodomys ordii parvabullatus_; and _Sigmodon hispidus solus_. Other characteristic mammals of this Province in the state of Tamaulipas are: _Sylvilagus floridanus connectens_; _S. audubonii parvulus_; _Lepus californicus merriami_; _Perognathus merriami merriami_; _Dipodomys ordii compactus_; _Orzomys melanotis carrorum_; _Reithrodontomys fulvescens intermedius_; _Peromyscus boylii ambiguus_; _Canis latrans texensis_; _C. l. microdon_; _C. lupus monstrabilis_; _Taxidea taxus berlandieri_; _Mephitis mephitis varians_; _Felis pardalis albescens_; _Trichechus manatus latirostris_; and _Odocoileus virginianus texanus_. Many other kinds of mammals occur mainly in the Tamaulipan Province but are not listed above because they occur also in one or more of the other provinces.