It is not easy to measure the _width_ of the ears in these animals: upon measuring with a thread over the curve of the outer side I have found the width of the ears of the present animal to be as above given,—the dimension slightly exceeding that stated in the Proceedings of the Zool. Soc. Footnote 25: Ρειθρος, a channel; Οδον, a tooth. Footnote 26: The tail is imperfect. Footnote 27: They are naked, but I suspect the hair has been rubbed off. Footnote 28: I am acquainted with seven North American Species of _Muridæ_, all of which possess the dentition of _Hesperomys_. Footnote 29: Ἐσπερος, West, and Μυς. Footnote 30: I am acquainted with only one exception, and that is in the genus _Castor_. In the genus _Ondatra_, the descending ramus is but slightly twisted outwards, but in all the other _Arvicolidæ_, whose crania I have examined, it is remarkably so, and in the genera _Spalax_ and _Geomys_, where this character is carried to the extreme, the descending ramus projects from the alveolus of the long inferior incisors, in the form of a rounded and almost horizontal plate. Footnote 31: In aged individuals of some of the species of _Arvicolidæ_, the molar teeth possess short roots. In a skull of _Ondatra_ now before me I find all the molars divided at the base into two portions, which in all probability would have formed solid roots had the animal lived longer. Footnote 32: See Proceedings of the Zoological Society for April 9th, 1839, p. 61. Footnote 33: Azara’s Voyages dans l’Amerique Meridionale, vol. i. p. 324. Footnote 34: Transactions of the Zoological Society, vol. ii. p. 84. Footnote 35: Azara Voyage dans l’Amerique Meridionale, vol. i. p. 324. Footnote 36: Compendio de la Hist. Nat. del Reyno de Chile, vol. i. p. 343. Footnote 37: Ἁβρος, soft; Κομη, hair. Footnote 38: There is a wide difference between the present animals and the _Arvicolidæ_ in the form of the occipital condyles: the same difference is also observable between _Echimys_ and _Mus_. The _Octodontidæ_ 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 _Leporidæ_. Footnote 39: I have not had an opportunity of examining the skull of _Abrocoma Bennettii_. Footnote 40: This nail no doubt is used to cleanse the fur, and the bristly hairs may also assist in the operation; the two small toes of the Kangaroo’s hind foot are used for the same purpose. Footnote 41: The skull is, unfortunately, imperfect, the hinder portion is injured, and the arches which enclosed the ant-orbital openings are broken. Footnote 42: Azara ‘Voyages dans l’Amerique Meridionale,’ vol. i. p. 316. Footnote 43: See Proceedings of the Zoological Society for April, 1839, p. 61. Footnote 44: Azara, Voyage dans l’Amérique Méridionale, vol. i. p. 318. Footnote 45: