The average annual temperature exceeds 18° C. but the mean of the coolest month is less than 18° C. This sub-climate is characterized by a short rainy season in summer and occurs on the west side of the southern part of the Sierra Madre Oriental and on the Mexican Plateau; it occurs also in the area northwest of Reynosa and on the east side of the Sierra Madre Oriental but in these areas the rainfall is irregularly distributed in the year. _Cool_ The average annual temperature is less than 18° C. but the mean of the warmest month exceeds 18° C. This sub-climate occurs only on the west side of the northern part of the Sierra Madre Oriental. Moderate Rainy Temperature Climate (Clima Templado Moderato Lluvioso) This type of climate is characterized by the coolest month having a temperature of between -3° and 18° C. In the northeastern and central parts of Tamaulipas, including the Sierra de Tamaulipas, Ciudad Victoria, Gómez Farías, Rancho Pano Ayuctle, and Llera, the average temperature of the warmest month is less than 22° C.; the winters are dry and not rigorous, and the wettest month has ten times as much rain as the driest. In the Sierra San Carlos the average temperature of the warmest month is less than 22° C., and the rainy season is in the autumn. Tropical Rainy Climate (Clima Tropical Lluvioso) This climate is characterized by the average temperature of all months being above 18° C. and the mean-annual rainfall being above 75 cm. According to the distribution of precipitation this type of climate can be divided into: (1) areas having periodic rain and wet winters (southeastern Tamaulipas, south of 22° north latitude and east of 99° west longitude), and (2) areas having an irregular rainy season and dry winters (area around Ciudad Mante, between 99° 30´ and 98° 30´ west longitude and south of 22° 30´ north latitude). AFFINITIES OF TAMAULIPAN MAMMALS Owing to the differences in climate from one region to another, the flora and fauna also differ, especially in the southern part of the state as compared with the northern part. For expressing the taxonomic resemblance of mammalian faunas having nearly equal numbers of taxa, Burt (1959:139) recommended the following formula: C × 100/(N_{1} + N_{2} - C) (where C is the number of taxa common to the two faunas, N_{1} is the number of taxa in the smaller fauna, and N_{2} is the number of taxa in the larger fauna). For non-flying mammals the resemblance of the Tamaulipan fauna to that of Texas, adjacent to the north, and Veracruz, adjacent to the south, is as follows: _Genera._--Texas 65 per cent, Veracruz 60 per cent. _Species._--Texas 45 per cent, Veracruz 39 per cent. For bats the resemblance of the Tamaulipan fauna to those of Texas and Veracruz is as follows: _Genera._--Texas 40 per cent, Veracruz 51 per cent. _Species._--Texas 24, Veracruz 39.