200 Mr. G. R. Waterhouse on the Rodentia. 



tural History' and in the ' Annals/ it appears to me that there 

 are some which are open to objection. I cannot feel a doubt 

 but that the several genera associated under the heads Sciu- 

 ridce and Muridce are naturally grouped. On the other hand, 

 I think upon more mature consideration that the group Arvi- 

 colidce requires revision. In the first place, I do not consider 

 it as a group of equal value with the Sciuridce or Muridce*. 

 The genera Ondatra, Arvicola and Lemmus of authors are 

 undoubtedly most closely allied ; indeed the first and last of 

 these so-called genera might perhaps, without impropriety, 

 be regarded as subgenera or sections of the genus Arvicola. 

 The animals composing these groups have all the essential 

 characters of the Muridce, but differ in having rootless molars 

 and in the form of the lower jaw. They have, moreover, some 

 peculiarities in the structure of the cranium, which have been 

 pointed out. 



Here all the characters alluded to are combined, with three 

 true molars, the normal number in the Muridce, and may be 

 conveniently used to define the Arvicolina as a subfamily of 

 that group. In my paper on the Arvicolidce I had placed in 

 that section, besides the three genera above mentioned, two 

 others, Ascomys and Castor. These genera M. Gervais is 

 of opinion should be arranged, — the former next the Sciuridce, 

 and the latter in that family. According to Prof. Wagner, 

 the genus Castor is associated with Myopotamus under the 

 family title Castorina, and the genus Ascomys forms part of 

 his family Cunicularia, — a family which, in my opinion, is 

 made up of various groups of Rodents : they all burrow in 

 the ground, and being fitted for that habit, they have a certain 

 superficial resemblance, as is also the case with the Myopo- 

 tamus and the Beaver, both being aquatic animals f. With 

 M. Gervais' views I am more inclined to agree : the two genera 

 under consideration are undoubtedly members of the great 

 Murine section, and certain points of resemblance in the form 

 of the skull and lower jaw induced me to place them in the 

 Arvicoline group. In the bony palate of Ascomys and Arvi- 



* See the ' Annals of Natural History* for October 1841, vol. viii. p. 83. 



| Among the Rodents there are five truly aquatic species, (besides others 

 which take to the water, more or less,) the Beaver, the Ondatra, the Hydro- 

 viys, the Myopotamus and the Hydrochoeres ; the second belongs to the 

 Arvicolina ; the third is nearly allied to Mus ; the fourth approaches closely 

 to the genera Echimys and Capromys ; the last is essentially a Cavy, whilst 

 the first differs from either. In nearly every family of Rodents there are 

 burrowing species, and many possess aquatic, climbing and burrowing 

 species. Such facts are most common in various groups of animals. 



