OBSERVATIONS ON THE RODENTIA. 593 



Art. V. — Observations on the Rodentia, with a view to point out 

 the groups as indicated by the structure of the Crania, in this Or- 

 der of Mammals. By G. R. Waterhouse, Esq., Curator to the 

 Zoological Society, Vice-Pres. of the Entomological Society. 



( Continued from page 279.) 

 Family V. — Arvicolidje. 



Dentition. — Incisors as broad as deep, nearly cylindrical : 

 molars ■§-§■> or ii> rootless. 



Skull. — Ant-orbital opening of moderate size, or small ; 

 anterior root of the zygoma thrown up from the plane of the 

 palate : temporal bone produced anteriorly and laterally, and 

 encroaching on the temporal fossa? : palate more or less con- 

 tracted in front, the inter-molar portion descending more or 

 less below the level of the anterior portion. 



Lower jaw. — Coronoid process large (usually very large) ; 

 articular surface of the condyles broad, — in some species with 

 the transverse diameter equal to the longitudinal : descend- 

 ing ramus with the angles twisted outwards, and situated 

 above the plane of the crowns of the molar teeth. ■ 



The genera Castor, Ondatra, Arvicola, Lemmus, Geomys and Spalax, 

 belong to this family. 



A transverse section of an incisor tooth, in Arvicola presents 

 a nearly circular figure (fig. 70,) and in this respect differs 

 from Mus, in which the incisors {m) are almost always com- 

 pressed and deeper from front to back, and where the sides 

 and front are nearly flat. In the molar teeth in the present 

 family the folds of the enamel generally divide the tooth into 

 angular-shaped portions, as represented in the figure of the 

 skull of Ondatra, and these teeth are rootless, and continue 

 to grow at the base as they wear away at the opposite extre- 

 mity ; but in aged individuals the supply of pulp decreases, 

 and the base of the tooth begins to divide into two or three 

 false fangs, as in fig. 70, k, which represents a molar tooth of 

 very old specimen of the Muskwash. I say false fangs, for 

 these roots are of an irregular form and unlike the true fangs 

 of the rat's molars. 



1 The only rodents I am acquainted with, besides the Arvicolidce, in 

 which the descending ramus of the jaw is thus raised, are those belonging 

 to the genus Cricetus, but here this process is of the same form as that of 

 the rat, and the space occupied by the molar teeth is remarkably small, 

 whereas in the Arvicolidce it is great. 



3 T 



