76 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



same Plate— recede still farther from the genus Mm, and approach more nearly 

 (as regards the dentition) to the Arvicolidce. Among the species here described I 

 may mention as examples, M. griseoflavus, M. zanthopy gus , 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 



Neotom 





The skulls of the animals belonging to the genera Castor, Ondatra, Arvi- 

 cola, Spalax, and Geomys, which constitute the principal groups of the family 



Muridce 





others, the following distinctive characters. 



The temporal fossa 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 

 less deep from front to back — th 

 molar teeth are rootless.f and t 



Muridce 



and 



Mus. The 



Mus 



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. 



Hesp* 



Muridce 



* 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. 



