_Dentes primores ²⁄₂ acuti, eradicati, anticè læves: molares utrinque ⁴⁄₄ subæquales, illis maxillæ superioris in areas duas transversales ob plicas vitreas acutè indentatus divisis; plicis utriusque lateris vix æquè profundis; illis mandibulæ inferioris in tres partes divisis, plicis vitreis his internè, semel externè indentatis, areâ primâ sagittæ cuspidem fingente, cæteris acutè triangularibus._ _Artus subæquales._ _Antipedes 4–dactyli, externo brevissimo, intermediis longissimis et ferè æqualibus._ _Pedes postici 5–dactyli; digito interno brevissimo. Ungues breves et debiles, illo digiti secundi lato et lamellari; omnibus setis rigidis obtectis._ _Caput mediocre, auribus magnis, membranaceis; oculis mediocribus._ _Cauda breviuscula._ _Vellus perlongum, et molle._ The genus _Abrocoma_ is evidently allied on the one hand to the genera _Octodon_, _Poephagomys_, and _Ctenomys_, and on the other to the family _Chinchillidæ_. The four genera just mentioned possess so many characters in common, that it would be well to unite them, and the name _Octodontidæ_ may be used to designate the group. The _Octodontidæ_ appear to bear the same relations to _Echimys_, as the _Arvicolæ_ do to the _Muridæ_. 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_.