RODENTIA. 459 



deposits does not appear to be specifically separable from the 

 existing Beaver (Castor fiber]. The great Trogontherium (fig. 

 372) of the Post-Tertiary de- 

 posits of Europe, also appears 

 to be generically inseparable 

 from Castor. The Castoroides 

 Ohioensis of the Post-Tertiary 

 period of North America 

 seems to be rightly referred 

 to a separate genus. The 

 only known species attained 



J i . Fie. 372. J aw of i rogon thenum C nviert. 



a comparatively gigantic size, Post-Pliocene, 



reaching a length of about 



five feet. The Chalicomys of the European Miocene and 

 Pliocene deposits appears to be nearly related to the Beavers ; 

 and the bone-caves of Brazil have yielded a species of Myopot- 

 amus nearly allied to the existing Coypu (Myopotamus coypus) 

 of South America. 



Fam. 5. Muridcz. The fifth family of Rodents is that of the 

 Murida, comprising the Rats, Mice, and Lemmings. In this 

 family the tail is long, always thinly haired, sometimes naked 

 and scaly. The lower incisors are narrow and pointed, and 

 there are complete clavicles. The hind-feet are furnished with 

 five toes, the fore-feet with four, together with a rudimentary 

 pollex. 



Remains of Rats and Mice have been obtained in the Mio- 

 cene and Pliocene Tertiary of Europe, and the Post-Tertiary 

 forms are in several cases very nearly if not altogether un- 

 distinguishable from existing forms. The genus Cricetus, com- 

 prising the existing Hamster, is represented in Post-Tertiary 

 deposits by a form probably identical with the living C. vulgaris. 

 The Lemmings (Myodes) are represented by at least one species 

 in Post-Tertiary deposits in Britain, occurring after the Glacial 

 period, and being contemporary with " palaeolithic" man. The 

 Voles or Campagnols (Arvicola) commence in the Pliocene, 

 and are abundantly represented in Post-Tertiary deposits. The 

 Post-Glacial deposits of Britain have yielded remains of the 

 Arvicola pratensis, A. agrestis, and A. amphibia, the last of 

 which (the well-known " Water-rat") occurs also in Prae- 

 Glacial accumulations. 



Fam. 6. Dipodidce. The sixth family of the Rodents, which 

 is sufficiently important to need notice, is that of the Dipodidcz 

 or Jerboas, mainly characterised by the disproportionate length 

 of the hind-limbs as compared with the fore-limbs. The tail 

 also is long and hairy, and there are complete clavicles. 



