Note 30 — Genus—REITHRODON.[25]

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.