196 PHENACODUS CHAP. 
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* is highly 
remarkable. The tail 
too, an organ which is 
long in some of the 
early forms, gets short 
in their modern deriva- 
tives. . 
Coupled with the in- 
creasing perfection of the 
foot as an organ used 
merely for the support 
of the body, certain in- 
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- 
(After Osborn. ) 
1 
12 
x 
Phenacodus primaevus. 
Fia. 110.—An early Ungulate. 
' This creature is, however, sometimes referred to the neighbourhood of the 
Rodents. 
