364 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



era Sciurus and Myoxus respectively, while Plesiarctomys presents 

 us with an ancestral form of the true marmots. Among the extinct 

 genera we find some remarkable suggestions of marsupial structure ; 

 thus, Pseudosciurus and Sciurodon, in the character of their den- 

 tition, approach the Australian koala (Phascolarctos cinereus), and 

 Sciuroides recalls the phalangers and the kangaroo-rats (Hypsi- 

 prymnus). 



The Miocene rodents, principally represented in France and 

 Germany, although still retaining a number of the older types, as 

 Theridomys, show a much closer approximation to the modern 

 fauna. Among living genera we find (in the newer deposits) repre- 

 sentatives of the true squirrels, dormice, porcupines, and pikas or 

 tailless hares (Lagomys). The rabbits or hares (Lepus) are en- 

 tirely wanting, as, indeed, they are from the whole of the Euro- 

 pean Tertiary series, and likewise the true mice (Mus), if we ex- 

 cept the single species, Mus (Acomys) Gaudryi, from Pikermi, 

 Greece. The genus Cricetodon, whose earliest appearance is in 

 the Oligocene deposits of France, was without doubt the near 

 ally of the hamsters (Cricetus). One or more species of marmot 

 (Arctomys) have been indicated as belonging to the Miocene de- 

 posits of both France and Germany, but it is a little doubtful 

 whether the horizons whence the remains were obtained have 

 been correctly identified. Arctomys primigenia, from Eppelshcim, 

 is not improbably a comparatively recent species, and not impos- 

 sibly identical with either A. marmotta or A. bobac. The most 

 abundant forms of this period are Myolagus Meyeri and Steneofiber 

 Jageri, both from the Upper Miocene of Germany and France, the 

 last replacing the more modern true beavers (Castor). Of the 

 older Miocene genera ArchaBomys stands intermediate between the 

 still earlier Protechimys and the chinchillas, and Issiodoromys be- 

 tween Nesokerodon and the cavies. The Pliocene rodent fauna 

 does not differ essentially from the Upper Miocene, of which it 

 may be considered to be a mere amplification. The recent genera 

 occurring here are already in the main represented in the period 

 preceding, although a limited number of new types, such as the 

 beaver (Castor Issiodorensis, possibly identical with the recent 

 Castor fiber; Puy-de-D6me, France) and the Arvicolida3, are for 

 the first time introduced. The forms related to the South Ameri- 

 can fauna have, on the other hand, completely disappeared from 



