84 
ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 
In the Octodontidce the skull is rather short, the inter-orbital space is broad ; 
the ant-orbital passage is large ; the zygomatic arch is thrown out horizontally 
from the plane of the palate ; the malar bone is broad and somewhat compressed, 
and throws up a small post-orbital process ; the glenoid cavity of the temporal 
bone is narrow ; the palate is contracted, and deeply notched posteriorly, the 
portion which lies between the molar teeth descends below the level of the anterior 
portion ; the incisive foramina are wide : the body of the anterior and posterior 
sphenoids is very narrow; and the foramina on either side of them are large : the 
occipital condyles are very narrow, widely separated, and the articular surface 
is nearly vertical.* The descending ramus of the lower jaw springs from the 
outer side of the alveolar portion, and terminates in a point, more or less acute. 
The incisors of the upper and lower jaws are of the same width : the molars 
are rootless. 
In external characters the species of the present group vary considerably. 
The toes are 5 1 5 or 4 j 5. The claws of the hind feet are covered by strong, curved 
bristly hairs. 
The principal points of distinction in the external characters of the four 
genera under consideration, may be thus expressed. 
t TOES 5 1 5. 
A. Fore feet formed for burrowing — strong and armed with large claws ; tail short. 
a. Ears minute, incisors very broad ....... Ctenomys. 
b. Ears small, incisors broad ......... Poephagomys. 
B. Fore feet weak ; claws small ; incisors narrow ; ears large. 
a. Tail with the apical portion furnished with long hair . , . . Octodon . 
ft TOES 4 1 5. 
b. Tail furnished throughout with short adpressed hairs .... Abrocoma. 
It is not only in the comparatively small size and weakness of the fore feet 
that Abrocoma approaches more nearly to Octodon ; but it agrees, in having the 
soles, both of the fore and hind feet (which are devoid of hair), covered with minute 
round fleshy tubercles (see the under side of the tarsus figured in Plate 28.) 
In Octodon, however, the toes have on their under side transverse incisions, 
as the Muridce, and many other Rodents ; a character not found in Abrocoma. 
* There is a wide difference between the present animals and the Arvicolidce in the form of the occipital 
condyles : the same difference is also observable between Echimys and Mus. The Octodontidce in fact have the 
same form of condyles as the Chinchillas and Cavies. In this and many other characters the last mentioned 
animals evince an affinity to the Leporidce. 
