ORDER RODENTIA. 1417 



of the order, and are mainly characterised by certain peculiar features 

 of the skull. With the exception of Sminthus, premolars are want- 

 ing ; and the true molars, except in the Australian Hydromys and 



2 x 



Xeromys, where they are reduced to -, are - in number; while 



2 3 



the lower incisors are laterally compressed. The molars may be 

 rooted or rootless, and either brachydont and tuberculate, or hypso- 

 dont with re-entering enamel-folds. This family may be divided 

 into several subfamilies. Of these the Cricetince have the cusps of 

 the upper molars arranged in two longitudinal series ; these teeth 

 being either rooted or rootless. This subfamily, which is now the 

 dominant one in America, but was formerly largely developed in 

 the Old World, where it is still found, appears to represent the 

 archaic or generalised type of the family. In the Voles, or the 

 more specialised Cricetines, the cusps of the molars have become 

 modified into triangular prisms alternately arranged (fig. 1294), and 



Fig. 1294. — The left upper and lower molars of the Water-Vole {Arvicola amphibius). 



Enlarged. 



the roots are generally not developed, so that the crowns are hypso- 

 dont. Siphneus, which connects this family with the preceding, is 

 found at the present day in Central Asia, and is represented by a 

 living species in the Pleistocene of the Altai, and by another, which 

 is extinct, in the Pliocene of Northern China. Of Arvicola (Microtus 1 ), 

 and Lemmus (including Cuniculus), there are numerous species in 

 the Pleistocene of Europe, some of which are identical with living 

 forms, while others are extinct ; the former genus being also repre- 

 sented in the Forest-bed and the Coralline Crag. The allied Fiber, 

 of North America, occurs also in the Pleistocene of the same 

 country. In the typical or less specialised forms, the molars usually 



1 This name, as being earlier than Arvicola, is adopted by several recent 

 writers. 



