76 
ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 
same Plate — recede still farther from the genus Mus, and approach more nearly 
(as regards the dentition) to the Arvicolidce. Among the species here described I 
may mention as examples, M. griseojlavus, 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 Arvicolidce 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 Arvicolidce 
and Neotoma. 
The skulls of the animals belonging to the genera Castor, Ondatra, Arvi- 
cola, Spalax, and Geomys, which constitute the principal groups of the family 
Arvicolidce, when compared with those of the family Muridce, present, among 
others, the following distinctive characters. 
The temporal fossce are always much contracted posteriorly, by the great 
anterior and lateral development of the temporal bones ; the plane of the inter- 
molar 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 * directed 
outwards from the plane of the horizontal ramus. The incisor teeth of the Arvi- 
colidce differ from those of the Muridce in being proportionately broader and 
less deep from front to back — they are not laterally compressed as in Mus. The 
molar teeth are rootless,! 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 Muridce, and do not 
* 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 Arvicolidce, 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. 
+ In aged individuals of some of the species of Arvicolidce, 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 proba- 
bility would have formed solid roots had the animal lived longer. 
