[27]. They are naked, but I suspect the hair has been rubbed off.
[28]. I am acquainted with seven North American Species of Muridæ, all of which possess the dentition of Hesperomys.
[29]. Ἐσπερος, West, and Μυς.
[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.
[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.
[32]. See Proceedings of the Zoological Society for April 9th, 1839, p. 61.
[33]. Azara’s Voyages dans l’Amerique Meridionale, vol. i. p. 324.
[34]. Transactions of the Zoological Society, vol. ii. p. 84.
[35]. Azara Voyage dans l’Amerique Meridionale, vol. i. p. 324.
[36]. Compendio de la Hist. Nat. del Reyno de Chile, vol. i. p. 343.