374 MAMMALIA. 



growing from persistent pulps throughout life. To admit of 

 the gnawing action of the mandible, its articulation with the 

 skull is antero-posteriorly elongated. The masseter muscle is 

 extended forwards at its upper end, often through an enlarged 

 infraorbital foramen at the anterior extremity of the zygomatic 

 arch, sometimes over the outer face of this arch ; while its 

 insertion upon the mandible usually extends forwards in a 

 similar manner well below the tooth-bearing border. The jugal 

 bone occupies the middle part of the zygomatic arch, and the 

 orbit is never separated by bone from the large temporal fossa. 

 The brain is of a very primitive type, the cerebral hemispheres 

 being smooth and not extending backwards far enough to cover 

 any part of the cerebellum. 



The last-mentioned character, among others, suggests the 

 great antiquity of the rodent order ; and Palaeontology indeed 

 confirms this suggestion, for the skulls of typical, fully-differen- 

 tiated rodents are known as low down in the Tertiary series as 

 the Middle Eocene, at least in North America. No essential 

 change seems to have occurred among the representatives of 

 the order since the latter part of the Eocene period, either in 

 the Old or New World. It may perhaps be stated that among 

 the dominant types at the present day the reduction of the 

 premolars and the complication of the molars are more common 

 features than they were in earlier periods; otherwise it is 

 difficult to recognize evidences of evolution in any direction. 



Sub-Order 1. Tillodontia. 



The only known group of early Tertiary mammals which 

 can be claimed with much probability as connected with the 

 direct ancestors of the Rodentia, is that of the TILLODONTIA. 

 This is represented by Tillotherium in the Middle Eocene of 

 Wyoming, and perhaps also by other genera in the Lower 

 Eocene both of America and Europe. Here the rodent-like 

 skull, with the form of the cerebral cavity denoting a primitive 

 type of brain, still retains traces of the canine teeth, with most 

 of the premolars ; and the incisors are just beginning to assume 

 their powerful gnawing functions. 



