_Baiomys taylori paulus_, Miller, Bull. U. S. Nat. Mus., 79:137, December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:317, April 29, 1924 (part); Burt, Miscl. Publ., Mus. Zool., Univ. Michigan, 39:54, February 14, 1938; Goldman, Smith. Miscl. Coll., 115:373, July 31, 1951 (part); Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955 (part); Hooper, Occas. Papers Mus. Zool., Univ. Michigan, 565:13, March 31, 1955; Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959 (part). _Baiomys musculus musculus_, Goldman, Smith. Miscl. Coll., 115:336, July 31, 1951 (part). _Type._--Adult male, skin and skull; No. 62075, University of Kansas, Museum of Natural History; 1 mi. S Pericos, Sinaloa, Republic of Mexico; obtained on June 14, 1954, by A. A. Alcorn, original number 1754. _Range._--Central Nayarit northward through western Sinaloa, to as far north as south-central Sonora, see Figure 11. Zonal range: Lower arid tropical, closely approximating the Sinaloan Biotic Province of Goldman and Moore (1945:349). Occurs from near sea level at Escuinapa (43 feet), Sinaloa, to 3200 feet at a place 2 mi. WNW Tepic, Nayarit. _Diagnosis._--Dorsal ground color Buffy Brown (some specimens near Olive Brown); proximal fourth of individual guard hairs of dorsum black-tipped, distal three-fourths dark grayish; dorsal underfur black-tipped having subterminal band of Buffy Brown; hair around eyes buffy to base; belly Pallid Neutral Gray with overtones of buff; individual hairs in region of chin whitish-gray to bases; vibrissae blackish to bases except ventralmost, those being white to base; tail Dark Olive above, slightly paler below. Average and extreme external measurements of 13 adults from 15 mi. N Rosario, Chele, Sinaloa, 300 ft., are as follows: Total length, 109.6 (99-120); length of tail, 43.4 (38-49); length of body, 66.2 (58-75); length of hind foot, 11.2 (10-12). Average and extreme cranial measurements of 19 adults from the same place are as follows: occipitonasal length, 18.2 (17.7-18.9); zygomatic breadth, 9.6 (9.2-10.1); postpalatal length, 6.9 (6.5-7.3); least interorbital breadth, 3.6 (3.4-3.8); length of incisive foramina, 3.9 (3.5-4.2); length of rostrum, 5.9 (5.5-6.6); breadth of braincase, 8.7 (8.3-8.9); depth of cranium, 6.5 (6.2-6.7); alveolar length of maxillary tooth-row, 3.1 (3.0-3.2); breadth of zygomatic plate, 1.8 (1.6-2.0); for photographs of skull, see Plate 2_c_, and Plate 4_d_.