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 3Ius decu-
manus, here pointed out, it will be perceived, there is an approach made to the
ArvicoUdce.
The dimensions of the skull (so far as an imperfect specimen will allow' of
their being taken) are as follows :—
Distance between front of incisors, (upper jaw ) and the first molar tooth 0 8
Longitudinal extent of the three molars on either side, taken tggether . 0 4^-
Length of nasal b o n e s ........................................................................................0 7^
-----------of incisive f o r a m i n a ..............................................................................0 4^
“Width between orbits . . . . . . . . . 0 2 ^
Length of of lower jaw . . . . . . . I ij
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
amphihia.y’—D.
When at Paris I examined what I believe to be the original Miis Braziliensis,
since the specimen was labelled ''R a t de B razil 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 3Ius Braziliensis, since it is
confounded by Desmarest, Fischer and Lesson with the R a t troisième or R at
Angouya of Azara, which I believe to be a very different animal. The description
given by the authors just mentioned are taken from Azara, who gives the
following characters to distinguish the Rat Angouya: “ Du museau à la queue, et
sur les côtés 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
inférieure de l’animal est blanchâtre, plus claire sous la tête, et plus foncée entre
les jambes de devant ; le pelage est doux, très-serré, et le poil, qui est à la racine
de l’oreille, cache le conduit de celle-ci.”