78 MR. E. T. BENNETT ON THE GENUS OCTODON. 
of Poephagomys, it is not in my power to pursue further a close comparison between 
the whole of these three genera. But a few general remarks on them may be offered ; 
and the comparison may afterwards be limited to that which can be made between the 
two of which examples are now before me. 
The general form of the animals of these several groups bears a close resemblance to 
that of the Water Rat, Arvicola amplibia, LaCép. The compactness of the body is the 
same ; the head is equally distinguished for its shortness and rotundity ; the limbs are 
of moderate length ; and the tail, covered like that of the Water Rat with short adpressed 
hairs, is, similarly with the tail of that animal, of less length than the body. In the 
proportionate length of the latter member there exists, however, a considerable differ- 
ence among the types of the three genera: in Octodon the length of the tail is more 
than one half of that of the body and head taken together ; in Ctenomys it is rather 
more than a third; in Poephagomys, according to M. F. Cuvier’s admeasurements, it 
is exactly one third. Another external difference is observable among them in the 
size of the outward ear: in Octodon and in Poephagomys the auricle is of moderate size 
and distinctly visible, its length being about equal to the distance interposed between 
its base and the eye; in Ctenomys it is so minute as to be concealed amid the surround- 
ing fur. A third external difference among them is of far more importance than either 
of the preceding, in as much as it is indicative of widely different habits: in Octodon and 
in Poephagomys the claws are rather long, moderately curved, and acute at their points, 
a form connected rather with arboreal than with terrestrial habits; in Ctenomys the 
claws are long, nearly straight, thick, and blunt, and have altogether the character 
which belongs to those of a burrowing animal. In all the three genera the toes are five 
in number on each foot: in Octodon and Poephagomys the claw of the inner toe on the 
fore foot is flattened and nail-like ; in Ctenomys it resembles that of the other toes in its 
strength, but is shorter and more curved. As the incumbent comb-like bristles sur- 
mounting the inner claws of the hinder toes are observable in both the animals before 
me, it is to be presumed that they exist equally in the other also. 
Between the crania of Octodon and of Ctenomys there are some differences in general 
form, resulting chiefly from the greater comparative length of the skull of the latter, and 
from the more ample development of its auditory appendages: the extent of the cellu- 
lar mass of bone connected with the internal organ of hearing bearing in these two 
animals an inverse ratio to that of the external ear; and the deficiency of auricle in 
Ctenomys being compensated for by the enlargement of the auditory cells. On account 
of the development of these cells in Ctenomys both laterally and posteriorly beyond the 
occipital ridge, the hinder portion of the craniwm acquires a breadth and squareness 
which afford a marked base, as it were, to the somewhat lengthened frustrum of a cone 
formed by the bones of the head and face: the greatest width of the entire mass is in 
Ctenomys at its hindermost part, while in Octodon the greatest width is at the external 
