378 MAMMALIA. 



genus Lagomys first occurs in the Upper Miocene of Oeningen, 

 Baden. 



Sub-Order 3. Simplicidentata. 



As already mentioned, most of the known rodents, both 

 recent and extinct, retain only one pair of incisors in the upper 

 jaw, exactly as in the mandible ; and these teeth in the recent 

 forms are completely chisel-shaped (scalpriform), their enamel 

 being nearly always confined to a band on the anterior face. 

 All the surviving SIMPLICIDENTATA (as these rodents are 

 termed) also seem to have lost the primitive articulation of 

 the fibula with the calcaneum in the hind limb. 



It is difficult to determine which are the least specialized 

 members of this great sub-order, but the squirrels and their 

 allies, or SCIUROMORPHA, occupy this position in some respects. 

 Their skull exhibits a slender zygomatic arch, in which the 

 jugal bone is not supported by any process from the maxilla, 

 exactly as in the primitive Ungulata. Their masseter muscle 

 is also so arranged that it does not necessitate any enlargement 

 of the infraorbital foramen. On the other hand, the insertion 

 of the masseter muscle on the mandible extends far forwards 

 on the side of the jaw below the teeth. So far as the skull 

 and dentition are concerned, the existing squirrel itself (Sci- 

 urus) seems to date back to the Upper Eocene in France 

 and Switzerland, and to the Lower Miocene (White River 

 Formation) in North America. The marmots, so far as known, 

 are quite modern, but during the Pleistocene period they had a 

 much wider range in Europe than at the present day. The 

 common souslik (Spermophilus) is represented in the Pleisto- 

 cene of England. The earliest known beaver is a small animal 

 (Steneofiber or Chalicomys) represented both by the skull and 

 limb-bones in the Miocene of Europe, and apparently with very 

 close allies in the Lower Miocene (White River Formation) of 

 North America. Castor itself first appears in the Upper Plio- 

 cene of Italy, France, and England ; and it had a much wider 

 range in Europe during the Pleistocene period than at the 

 present day. The animal does not appear to have been exter- 

 minated in Britain until about the twelfth century, and there 



