_B. t. paulus_ intergrades with _ater_ to the north in Chihuahua (see account of that subspecies), with _analogous_ to the south in Jalisco, and with _allex_ (see account of that subspecies) to the southwest in Nayarit and Jalisco. The zone of intergradation between _paulus_ and _analogous_ in Jalisco approximately borders the Rio Grande de Santiago from the western part of the State to the northwest shore of Lago de Chapala. Nineteen specimens from 2 mi. WNW Lagos de Moreno in northwest Jalisco seem to be intermediate between _paulus_ and _analogous_ in color, averaging slightly grayer than typical _paulus_. The series of 19 is referable to _paulus_ on the basis of cranial characters. A series of 34 specimens from 3 mi. W La Venta, Jalisco (referable to _paulus_), is indistinguishable in color of pelage from two series of _paulus_ from 5 mi. N Durango, and from 8 mi. NE of Durango, except that the antiplantar surfaces of the hind feet are sooty as in _analogous_. Seemingly, features of color mentioned above as diagnostic of the two subspecies are either present or absent and there is no tendency toward intermediacy in color in the population from 3 mi. W La Venta. The Rio Grande de Santiago may have acted in the past as a physical barrier reducing gene flow between _allex_ and _paulus_ and in separating completely the two populations for limited periods. _Specimens examined._--Total 176, all from the Republic of Mexico and distributed as follows: CHIHUAHUA: Rancho Sanignacio, 4 mi. S, 1 mi. W Santo Tomas, 1; El Rosario, 6700 ft., 1; 2 mi. W Minaca, 6900 ft., 11; Balleza, 1[50]. DURANGO: Rosario, 1[51]; type locality, 14[51] (including the type); _San Gabriel_, 2[51]; _Rancho Santuario_, 2[51]; 1 mi. N Chorro, 6450 ft., 1; _8 mi. NE Durango_, 6200 ft., 2; 5 mi. N Durango, 6400 ft., 2. ZACATECAS: Valparaiso, 6500 ft., 10[50]. AGUASCALIENTES: _18 mi. W, 2 mi. S Aguascalientes_, 6000 ft., 1; 16 mi. S Aguascalientes, 5[52]. JALISCO: 1 mi. NE Villa Hidalgo, 6500 ft., 1; 2 mi. WNW Lagos de Moreno, 6370 ft., 19; _2 mi. ESE Tequila_, 4000 ft., 11; _3 mi. W La Venta_, 33, 1[53]; _12 mi. W Guadalajara_, 3[54]; _Atemajac_, 12[50]; 4 mi. W Guadalajara, 5100 ft., 3; _2 mi. N, 1/2 mi. W Guadalajara_, 11; 2 mi. NW Magdalena, 4500 ft., 7[50]; _1 mi. N Tala_, 4400 ft., 3; 3 mi. W Tala, 4300 ft., 18. _Marginal records._--CHIHUAHUA: Rancho Sanignacio, 4 mi. S, 1 mi. W Santo Tomas; El Rosario; Balleza. DURANGO: Rosario, 6700 ft.; 1 mi. E Zarca (Blossom and Burt, 1942:1); 1 mi. N Chorro, 6450 ft. ZACATECAS: Valparaiso, 6500 ft. AGUASCALIENTES: 1 mi. N Chicalote (Blossom and Burt, 1942:4). JALISCO: 2 mi. WNW Lagos de Moreno, 6370 ft.; 4 mi. W Guadalajara, 5100 ft.; 3 mi. W Tala, 4300 ft.; 2 mi. NW Magdalena, 4500 ft. DURANGO: 5 mi. N Durango, 6400 ft.; type locality. CHIHUAHUA: 2 mi. W Minaca, 6900 ft. [50] United States National Museum (Biol. Surv. Collections). [51] American Museum of Natural History. [52] Univ. Illinois, Mus. Nat. History.