196 



PHENACODUS 



and not climbing or grasping organs ; the number of toes be- 

 comes reduced, and culminates twice (in the horse and in the 



Litopterna) in one toe 

 on each foot ; at the 

 same time the ulna be- 

 comes rudimentary and 

 fuses with the radius, 

 and the fibula in the 

 hind -limb undergoes a 

 like reduction. The 

 clavicle is absent even 

 in some of the oldest 

 types ; its presence in 

 ^ Typotherium l is highly 

 I remarkable. The tail 

 2 too, an organ which is 

 long in some of the 

 | early forms, gets short 

 | in their modern deriva- 

 ^ tives. 



Coupled with the in- 

 3 creasing perfection of the 

 foot as an organ used 

 %? merely for the support 

 * of the body, certain in- 

 1 teresting changes have 

 <^ taken place in the 

 "^ arrangement with re- 

 & gard to each other of 

 the several bonelets of 

 the wrist and ankle. It 

 has been held by Cope 

 and others that the truly 

 primitive disposition of 

 these bones was that pre- 

 sented to us by certain 



early types, such as Meniscotherium or the existing elephant or 

 Hyrax. In these animals there is (see Fig. 112) a serial arrange- 



1 This creature is, however, sometimes referred to the neighbourhood of the 

 Rodents. 



