The species of the genus _Hesperomys_, which depart most from the type—whose dentition is least like figs. 5, _a_, and 5, _b_, Plate 33., or 6, _a_, and 6, _b_, of the same Plate—recede still farther from the genus _Mus_, and approach more nearly (as regards the dentition) to the _Arvicolidæ_. Among the species here described I may mention as examples, _M. griseo-flavus_, _M. zanthopygus_, and _M. Darwinii_;—see the molar teeth figured in Plate 34. figs. 15, 16, and 17,—and among the North American species, those constituting the genus _Neotoma_. The latter make by far the nearest approach to the _Arvicolidæ_ of any which have yet come under my observation, not only in the dentition, but in the form of the skull and the large size of the coronoid process of the lower jaw; there is, nevertheless, a tolerably well marked line of distinction between the crania of the _Arvicolidæ_ and _Neotoma_. The skulls of the animals belonging to the genera _Castor_, _Ondatra_, _Arvicola_, _Spalax_, and _Geomys_, which constitute the principal groups of the family _Arvicolidæ_, when compared with those of the family _Muridæ_, present, among others, the following distinctive characters. The temporal _fossæ_ are always much contracted posteriorly, by the great anterior and lateral development of the temporal bones; the plane of the intermolar portion of the palate is below the level of the anterior portion; the coronoid process of the lower jaw is very large, the articular portion of the condyloid process is proportionately broad; the descending ramus, or posterior coronoid process, is so situated that its upper portion terminates considerably above the level of the crowns of the molars; this same process is generally[30] directed outwards from the plane of the horizontal ramus. The incisor teeth of the _Arvicolidæ_ differ from those of the _Muridæ_ in being proportionately broader and less deep from front to back—they are not laterally compressed as in _Mus_. The molar teeth are rootless,[31] and the folds of enamel are the same throughout the whole length of the tooth; whereas in _Mus_ they enter less and less deeply into the body of the tooth as we recede from the crown, and towards the base of the visible portion (the tooth being in its socket) the indentations of the enamel are obliterated. Now in the species of _Hesperomys_, the molar teeth are always rooted, and in the form of the skull and the lower jaw they agree with the _Muridæ_, and do not present the characters above pointed out as distinguishing the _Arvicolidæ_, and as regards the cranium and lower jaw, it is only in the genus _Neotoma_ that any approach is evinced.