This little animal was preserved in spirit, and has since been mounted, it is probable, therefore, that the colours have been slightly changed. It is of a smaller size than either of the preceding species. Its fur is long, extremely soft, and somewhat resembles that of the Chinchilla. The ears are smaller, and the tail is shorter, and less densely clothed with hairs than in _Reithrodon cuniculoides_. The skull (see Plate 43, fig. 20, _a_, 20, _b_, and 20, _c_,) differs in many respects from that of the species last mentioned. It is of a smaller size, the nasal portion is proportionately shorter and narrower, the incisive foramina are shorter; the pterygoid processes do not approximate so nearly at their base, and the pterygoid fossæ are very shallow, whereas in _R. cuniculoides_ they are deep. In the skull of the animal just mentioned there are two distinct longitudinal grooves on the palate, which extend backwards from the incisive foramina, and terminate in two rather large and deep excavations: these excavations are in the palatine bone, and situated between the last molar teeth; they are separated from each other by a narrow, longitudinal, elevated ridge; a narrow ridge also separates them from the pterygoid fossæ. At the bottom of each of these hollows are several minute foramina, and in front of them there are two larger longitudinal foramina. In _R. chinchilloides_, the longitudinal grooves on the palate and the posterior hollows are shallow, and consequently much less distinct; the pterygoid fossæ are very nearly on the same plane as the palate, and are indicated only by a very slight depression. The incisor teeth are broader than in _R. chinchilloides_, and the molar teeth are proportionately smaller. The thin plate which forms the anterior root of the zygomatic arch is deeply emarginated in front in _R. cuniculoides_ (see Plate 34, fig. 21, _b._); but in _R. chinchilloides_, the anterior margin of this plate is nearly straight, (see Plate 34, fig. 20, _c._) In the form of the lower jaw of the two animals under consideration there are differences which will be more clearly understood upon comparing the figures. I will therefore merely notice one remarkable character which is found in _R. cuniculoides_, and that is, that the condyloid process is rather deeply concave on the inner side, a character which does not exist in _R. chinchilloides_, nor do I recollect having observed it in any other Rodent. The principal dimensions of the skull of _R. chinchilloides_, are as follows:—