MAMMALIA. RODENTIA. 365 



forms have a very complex structure and mode of succession. 

 In some of the extinct forms, such as Mastodon and especially 

 Dinotkerium, the teeth are much more simple. In every case 

 the teeth have the same general structure, consisting of a 

 series of ridges of dentine, coated with enamel. In the more 

 specialised forms the valleys between the ridges are filled up 

 with cement. The acromion of the scapula has a recurved 

 process, similar to that often found in rodents. Clavicles are 

 absent. The radius and ulna are not ankylosed, but are 

 incapable of any rotatory movement. All the bones of the 

 extremities are very short and thick ; the scaphoid articulates 

 regularly with the trapezoid and the lunar with the magnum. 

 The ilia are vertically placed, and are very much expanded ; 

 the ischia and pubes are small, and form a short symphysis. 

 The femur has no third trochanter, and the tibia and fibula 

 are distinct. The fibula articulates with the calcaneum, and 

 the astragalus is very flat. 



Here brief reference may be made to the TiLLODONTiA 1 , a 

 group of extinct mammals found in the Eocene beds of both 

 Europe and North America. They seem to connect together 

 the Ungulata, Rodentia, and Carnivora. 



The skull resembles that of bears, but the grinding teeth 

 are of Ungulate type, while the second incisors resemble those 

 of rodents, and have persistent pulps. The femur has a third 

 trochanter, and the feet resemble those of bears in being 

 plantigrade and having pointed ungual phalanges, differing, 

 however, in having the scaphoid and lunar distinct. 



Order 5. RODENTIA. 



The Rodents form a very large and well-defined group of 



mammals easily distinguishable by their peculiar dentition. 



Canines are absent, and the incisors are very large and curved, 



growing from persistent pulps. They are rectangular in section 



1 See 0. C. Marsh, Amer. J. Set., 1875 and 1876. 



