90 
having the fore-feet strong and furnished with large claws formed 
for burrowing, and approaches most nearly, as regards its external 
characters, to the genus Poephagomys of F. Cuvier, (which appears to 
be the Psammoryctes of Poeppig,) inasmuch as its ears are of mode- 
rate size, whilst those of the species of Ctenomys are very small. 
The skull presents a very large antorbital opening, enclosed ex- 
ternally by the zygomatic process of the superior maxillary bone, 
the root of which is thrown out almost horizontally, and is on the 
same plane as the anterior palatal portion of the cranium: a second, 
very small opening, for the transmission of the infra-orbital nerve, is 
pierced through the root of the zygomatic process* ; the zygoma is 
deep and compressed; the portion of the palate situated between the 
molar teeth is contracted in front, and widest between the posterior 
molars; and the posterior portion of the palate presents a deep V- 
formed emargination. The descending ramus of the lower jaw, or 
that part which lies below and behind the alveolar portion, is thrown 
out from the outer side of the alveolus of the great inferior incisor, 
which extends almost to the condyle; its posterior portion is emar- 
ginated, and lower and hinder portion is produced in the form of an 
acute angle, terminating behind the vertical line, dropped from the 
condyle; the lower boundary of the descending ramus is produced 
externally and internally in such a manner, as when viewed from 
beneath, this part presents a horizontal platform of small extent, 
and which is broadest at about one-sixth of an inch from the angle of 
the jaw, the platform at this part being produced internally so as to 
form an obtuse angle. The condyloid portion of the lower jaw has 
a considerable antero-posterior extent, and the articular surface, 
which is rather broad, occupies rather more than half of this com- 
pressed condyloid process. The coronoid process is in the form of 
an isosceles triangle, the apex of which is of the same height as the 
condyle, or very nearly so. 
The molar teeth are rootless and four in number on each side of 
each jaw. 
These characters of skull and dentition all indicate the affinities 
of the animal under consideration with the Octodontide: the cranium 
compared with that of the known species of the family presents the 
modifications observable in the burrowing types, differing from the 
Octodons and Abrocomas, which live more on the surface of the 
ground and ascend trees and bushes, in the smaller size of the 
cranial cavity, and in the greater strength of all the parts, arising from 
the comparatively large size of the teeth, and more especially of the 
incisors, which are fitted for cutting rootst. The auditory bulle 
* This second small opening is found in the skull of Octodon, and appears 
to be represented in the figure (given by Mr. Bennett) in Ctenomys, though 
not so well expressed by the engraver as by the artist of the drawing from 
which the engraving was made. In Abrocoma there is no corresponding 
opening. 
+ The burrowing Sciuride and Muride, as compared with the typical 
examples of their respective groups, present a corresponding modification of 
the skull; the strength of the cranium is greater, and the cranial cavity 
