60 
ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 
extent of the symphysis menti. In the form of the incisors, the more contracted 
palate,, the great extent of the symphysis menti, and in fact in most of the points 
of dissimilarity, between the skull of the present animal and that of Mus decu- 
manus, here pointed out, it will be perceived, there is an approach made to the 
Arvicolidce. 
The dimensions of the skull (so far as an imperfect specimen will allow of 
their being taken) are as follows : — 
Fig. 3, c, Plate 33, represents the molar teeth of the upper jaw. Fig. 3, d, 
those of the upper jaw. 
“ This rat was caught at Bahia Blanca where the plains of Patagonia begin 
to blend into the more fertile region of the Pampas. It lived in holes amongst 
the tussocks of rushes, on the borders of a small, still, brook ; in its manner of 
diving and aquatic habits it closely resembled the English water-rat, ( Arvicola 
amphibia. )” — D. 
When at Paris I examined what I believe to be the original Mus Braziliensis, 
since the specimen was labelled “ Rat de Brazil St. Hilaire, 1818.” It agrees 
perfectly with the present animal excepting in being rather smaller, the length 
from the nose to the tail being 7 inches and 4 lines — the length of the tail is 7 
inches 9 lines, and that of the tarsus is 1 inch 11 lines; this difference in the 
length of the body may arise from difference of age, or even of sex. In the Paris 
Museum I saw what appeared to me to be a variety of the same species in which 
the under parts of the body are white. 
I have been minute in my description of the Mus Braziliensis, since it is 
confounded by Desmarest, Fischer and Lesson with the Rat troisieme or Rat 
Angouya of Azara, which I believe to be a very different animal. The descrip- 
tion given by the authors just mentioned are taken from Azara, who gives the 
following characters to distinguish the Rat Angouya : “ Du museau & la queue, et 
sur les cot^s du corps tout est brun-cannelle, parceque les poils ont une petite 
pointe cannelle ; puis, ils sont obscurs et enfin blanc vers las peau. Toute la partie 
infi£rieure de l’animal est blanchatre, plus claire sous la t6te, et plus fonc6e entre 
les jambes de devant ; le pelage est doux, tres-serr6, et le poil, qui est & la racine 
de l’oreille, cache le conduit de celle-ci.” 
of incisive foramina 
Width between orbits 
Length of ramus of lower jaw 
Distance between front of incisors, (upper jaw) and the first molar tooth 
Longitudinal extent of the three molars on either side, taken tggether 
Length of nasal bones ........ 
In. Lines. 
0 8 
0 41 
0 7i 
0 4.1 
0 21 
1 U 
