In the Octodontidæ 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.[[38]] 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 ⁴⁄₄=⁴⁄₄, rootless.
In external characters the species of the present group vary considerably. The toes are ⁵⁄₅ or ⅘. 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.
| † TOES ⁵⁄₅. | ||
| 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. | |
| †† TOES ⅘. | ||
| 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 Muridæ, and many other Rodents; a character not found in Abrocoma. Here the underside of the toes, like the sole of the foot, is covered with minute tubercles.
Though in the form of the skull Abrocoma Cuvieri[[39]] agrees most nearly with that of Octodon; it differs in having the anterior portion narrower and rather larger, compared to the part devoted to the protection of the brain; the zygomatic arch is shorter, the incisive foramina are longer, the body of the anterior sphenoid is narrower, and the auditory bullæ are larger. The principal differences observable in the form of the lower jaw of Abrocoma, when compared with that of Octodon, consists in the coronoid process being smaller, the condyloid narrower from front to back; the descending ramus more deeply emarginated posteriorly, and the angle longer and more attenuated.
In those characters in which the skull of Abrocoma departs from that of Octodon, it approaches nearer to Chinchilla. In the peculiar form and large size of the ears, in the extreme softness of the fur, in the greater development of the pads on the under side of the toes, and in the possession of only four toes to the fore feet, there are other points of resemblance between Abrocoma and Chinchilla. In the Chinchilla as well as in Octodon and Abrocoma, we find the toe corresponding to the second (counting from the inner side) furnished with a broad hollow nail;[[40]] there are also stiff bristly hairs covering this nail as in the Octodontidæ.
The extreme softness of the fur of the animals about to be described, suggested for them the generic name of Abrocoma. The fur consists of hairs of two lengths, and the longer hairs are so extremely slender that they might almost be compared to the web of the spider. The specific names applied are those of the distinguished naturalists who first made us acquainted with the two genera, Octodon and Poephagomys.