Our specimens generally agree in color with _S. truei_, but are larger than typical individuals and in this respect approach _S. sinaloensis_ of southern Sinaloa. Probably _truei_ and _sinaloensis_ both are only subspecies of the more southerly _S. colliaei_. The three nominal species currently constitute the _S. colliaei_ group in which the presence or absence of P3 seems to vary geographically. The tooth frequently is absent in the northern _truei_ and usually present (invariably in the specimens we have examined) in _colliaei_. Only one of our Sinaloan specimens is accompanied by a skull; in it P3 is present on the right side and absent on the left. External measurements of the male and two females are, respectively: total length, 512, 508, 504; length of tail, 263, 263, 252; length of hind foot, 64, 63, 64; length of ear from notch, 28, 29, 28. Cranial measurements of 61300 (a female) are: greatest length of skull, 56.2; zygomatic breadth, 32.6; interorbital constriction, 17.9; postorbital constriction, 17.9; length of nasals, 17.3; alveolar length of maxillary tooth-row (on side lacking P3), 10.9. _=Thomomys umbrinus atrovarius=_ J. A. Allen.--Two specimens (85104-05) from the valley of the Río Piaxtla, 1 mi. SE Camino Reál, 400 ft., resemble the description of _atrovarius_ and agree in size, color and most cranial details with a specimen (85744) from 5 mi. NW Mazatlán. The first-mentioned specimens extend the known range of the subspecies some 50 miles northward from Mazatlán (Bailey, 1915:96), and indicate the probable occurrence of the species at lower elevations in other parts of central Sinaloa. _=Peromyscus merriami goldmani=_ Osgood.--This subspecies has been reported previously only from the type locality, Alamos, Sonora. Eight specimens were collected in Sinaloa by W. L. Cutter in the autumn of 1957 as follows: 6 mi. N, 1½ mi. E El Dorado (75368-72); 2½ mi. N El Fuerte (75365-66); El Fuerte (75367). The first-mentional locality is approximately 200 miles south-south-east of the type locality. All specimens collected by Cutter were taken in lowland areas, supporting remarks by Commissaris (1960) concerning habitat preferences of _P. merriami_ as compared with those of the closely related _P. eremicus_. Two of three females from northeast of El Dorado were pregnant on November 18 and 19; one carried four embryos (8 mm. in crown-rump length) and the other three (11 mm.).