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, 

 I and the iibula in the 

 § hind -limb undergoes a 



like reduction. The 

 <, clavicle is absent even 



,;, in some of the oldest 

 C types ; its presence in 

 ^ Typotherium ^ is highly 

 E remarkal)le. The tail 

 •I too, an organ which is 

 J' long in some of the 

 'i early forms, gets short 



1 in their modern deriva- 

 ^ tives. 



o- Coupled with the in- 



-S creasing perfection of the 

 ", toot as an organ used 

 ^ merely for the support 

 ^ of the body, certain in- 

 1^ teresting changes have 

 o taken place in the 

 "^ arrangement with re- 

 S 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 i^re- 

 sented to us by certain 

 early types, such as Meniscothcrium 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. 



