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_. 1. ABROCOMA BENNETTII. PLATE XVIII. Abrocoma Bennettii, _Waterh._, Proceedings of the Zoological Society of London, for February 1837, p. 31.