MAMMALS OF THE MEXICAN BOUNDARY. 473 



Geographical range. — Inhabits the lower Sonoran Zone of the mes- 

 quite-wooded river bottoms from the Lower Gila River to the mouth of 

 the Colorado River. 



General characters. — One of the Largesl of the round-tailed species 

 (subgenus Neotoma), distinguished by its massive skull and teeth and 

 peculiar crania] characters from all of the species of the surrounding 

 region. The coloring is similar to that of Neotoma intermedia, being 

 paler than the typical form and darker than the desert phase of that 

 species (Neotoma intermedia gilva). The tail is more distinctly 

 bicolored, nearly black above, and quite copiously coated with rather 

 long hairs. It requires no close comparison wit h any of the neighbor- 

 ing forms, but is closely related to N. leucodon, described by Dr. 

 C. H. Merriam, from San Luis Potosi, Mexico. Mammae, 2 pairs. 



Color. — Upper parts grayish fulvous, thickly mixed with black- 

 tipped hairs, this color changing to gray on the legs and ochraceous buff 

 on the sides. Tail rather long-haired, almost black above, and white 

 below. Feet, a small patch at base of ear, and whole tinder surface of 

 body, white. Ears scantily clothed with gray and black, downy hairs 

 on concave surface, and on posterier two-thirds of convex surface: 

 anterior third of convex surface covered with longer black hairs. 

 Whiskers reaching to the shoulders; their color, mixed black and 

 white. Usually with a dusky orbital area. 



A young specimen (No. 6067S, U.S.N.M.), no larger than Onychomys 

 torridus, has a very silky coat, and is paler and more grayish than 

 adults. The middle of the back is smoke-gray, the sides drab-gray, 

 and the under parts white. The upper side of the tail is mouse-gray 

 instead of black. The convex surfaces of the ears are black ; but there 

 is no dusky orbital area. This animal was found attached to the teat 

 of its mother (No. 60677, U.S.N.M.), the latter dead in a trap. 



Measurements. — Length, 400 nun.; tail vertebra 1 , ISO; hind foot, 

 37; head, 55; car, from crown, 28. Skull, 47.5 by 23. 



Cranial characters. — The general shape of the skull in this species 

 (fig. 117) is suggestive of that of Neotoma leucodon Merriam. It has 

 the same prominent premaxillaries, ending posteriorly well behind 

 the nasals. The skull, as a whole, is considerably lower and more 

 convex antero-posteriorly than that of N. l( ucodon; and the brain-case 

 is much more Hat. The skull is more angular throughout, with the 

 zygomatic arches standing out more squarely anteriorly. The inter- 

 parietal bone has a less quadrate shape, being broader laterally, and 

 more pointed posteriorly. The frontal is broader interorbitally, and 

 lacks the prominent, almost beaded orbital rim. The nasals differ 

 markedly in shape from those of N. leucodon, in having a prominenl 

 swelling of their anterior extremities, which gives them a distinctly 

 spatulate form, in this respect agreeing with Sigmodon hispidus eremi- 

 cus, inhabiting the same region. The rostral portionof the skull is 



