MAMMALIA. 
73 
probable, therefore, that the colours have been slightly changed. It is of a smaller 
size than either of the preceding species. Its fur is long, extremely soft, and 
somewhat resembles that of the Chinchilla. The ears are smaller, and the tail 
is shorter, and less densely clothed with hairs than in Reithrodon cuniculoides . 
The skull (see Plate 43, fig. 20, a, 20, b , and 20, c,) differs in many respects 
from that of the species last mentioned. It is of a smaller size, the nasal portion 
is proportionately shorter and narrower, the incisive foramina are shorter ; the 
pterygoid processes do not approximate so nearly at their base, and the pterygoid 
fossae are very shallow, whereas in R. cuniculoides they are deep. In the skull 
of the animal just mentioned there are two distinct longitudinal grooves on the 
palate, which extend backwards from the incisive foramina, and terminate in two 
rather large and deep excavations : these excavations are in the palatine bone, 
and situated between the last molar teeth ; they are separated from each other by 
a narrow, longitudinal, elevated ridge ; a narrow ridge also separates them from 
the pterygoid fossae. At the bottom of each of these hollows are several minute 
foramina, and in front of them there are two larger longitudinal foramina. In 
R. chinchilloides, the longitudinal grooves on the palate and the posterior hollows 
are shallow, and consequently much less distinct ; the pterygoid fossae are very 
nearly on the same plane as the palate, and are indicated only by a very slight 
depression. The incisor teeth are broader than in R. chinchilloides, and the molar 
teeth are proportionately smaller. The thin plate which forms the anterior root 
of the zygomatic arch is deeply emarginated in front in R. cuniculoides (see 
Plate 34, fig. 21, b.) ; but in R. chinchilloides, the anterior margin of this plate is 
nearly straight, (see Plate 34, fig. 20, c.) 
In the form of the lower jaw of the two animals under consideration there are 
differences which will be more clearly understood upon comparing the figures. I 
will therefore merely notice one remarkable character which is found in R. cuni- 
culoides, and that is, that the condyloid process is rather deeply concave on the 
inner side, a character which does not exist in R. chinchilloides, nor do I recollect 
having observed it in any other Rodent. 
The principal dimensions of the skull of R. chinchilloides, are as follows : — 
In. Lines. 
Total length 12 
Width 0 8$ 
Length of nasal hones ............ 0 6^ 
of incisive foramina ........... 0 4 
Distance between the outer surface of the incisors and the first molar tooth, upper jaw . 0 4$ 
Longitudinal extent of the three molars of the upper jaw, taken together . . . 0 2| 
Length of a ramus of the lower jaw without the incisor ...... 0 8 
L 
