MAMMALIA. 
59 
colour of the ordinary fur, a dark brown tint ; chin, throat, chest, and 
rump, white ; the hairs covering the upper surface of the feet are of a dirty 
yellowish-white colour, and on the toes nearly white : ears densely clothed 
with longish hairs, those on the inner side chiefly of a deep golden colour, 
and those on the outer side brownish ; the ears are partially hidden by the 
long fur of the head ; tail sparingly clothed with hairs, above brown, and 
beneath brownish-white : the fur of the back is of a deep gray colour at the 
base, annulated with deep golden yellow near the apex, and blackish at the 
apex ; the longer hairs are black ; the hairs of the belly are pale gray at 
the base, and broadly tipped with golden yellow colour ; the white hairs 
on the throat, chest, and rump are of an uniform colour — not tinted with 
gray at the root ; — the hairs of the moustaches are black : the incisors of 
the upper jaw are of a deep orange colour, and those of the lower jaw are 
yellow : the thumb nail is truncated. 
In. Lines 
Length from nose to root of tail . .86 
of tail . . . . .79 
from nose to ear ... 1 8 
Habitat, Bahia Blanca, {September.) 
This species is nearly equal in size to the common rat {Mus decumanus). Of 
its skull * I possess but the anterior portion (see PI. 33. fig. 3, a. and 3, b.) : it 
appears to have been about the same size as that of M. decumanus , its proportions, 
however, are different : the nasal portion is broader and shorter, the ant-orbital 
outlet is rather smaller ; the plate, forming the anterior root of the zygomatic 
arch, and which protects this outlet, has its anterior edge distinctly emarginated, 
and not nearly straight as in M. decumanus , — the zygomatic arch is stouter, the 
space between the orbits is narrower, the palate is more contracted, the incisors 
are much broader, less deep from front to back, and have the anterior surface 
more convex ; the molar teeth are larger ; the lower jaw (see Plate 34. fig. 12, a.) 
when compared with that of Mus decumanus also offers many points of dis- 
similarity ; the principal differences consist in its greater strength, the com- 
paratively large size and breadth of the articular surface of the condyles, the 
upright position of the coronoid process — a perpendicular line dropt from the 
apex of which would touch the posterior part of the last molar — and the great 
* I am sorry to say the artist has not drawn this skull with his usual fidelity, a circumstance which I did not 
perceive until it was too late to make any alteration : it is too large, and the incisors are represented as project- 
ing forwards too much ; they are in the original so nearly at right angles with the upper surface of the skull 
that but a very small portion of them is seen, when it is viewed, as represented at fig. 3, a. 
In. Lines. 
Length of tarsus . . . . .20 
of ear . . . . . 0 
