THE TILLODONTIA 



507 



These animals have been regarded as ancestral Eodents, to 

 which the tooth characters just mentioned clearly show like- 

 nesses. The earliest known form is Estlionyx. This genus shows 



Fig. 2i7. ^TiUotherimnfodiens. Left lateral view of sl-ull. (From Flower, 

 after Jlar.sli.) 



such primitive characters, compared with its later representatives, 

 as the existence of all three pairs of incisors in the upper jaw, 

 but only two in the lower jaw. The enlarged incisors of both 

 jaws do not seem to have grown from persistent pulps. 



Anchippodvs, a later form, still preserves the upper pair of first 

 incisors in a vestigial form ; the strong second incisors grew from 

 persistent pulps. The most recent genus, Tillothermm, shows the 

 characteristics of the group at their height. The strong Eodent- 

 like, chisel -shaped incisors, which are reinforced by a small 

 additional pair in the upper jaws only, are persistent. The 

 grinding teeth are of the tritubercular pattern ; there are three 

 of each kind in the upper jaw, but in the lower jaw only two 

 premolars on each side. This is at any rate the case with some, 

 while others have three. The canine, though present in both jaws, 

 is insignificant. As in many ancient types, there is an entepi- 

 condylar foramen in the humerus. The feet were five-toed, and 

 bore sharp, laterally-compressed claws. The skull has been com- 

 pared in general aspect to that of a Bear. 



