56 
capitellum of the humerus. 
ELEMENTS OF MAMMALIAN ANATOMY. 
The styloid process projects 
from the distal end parallel with the process of the same 
name on the ulna. 
OR 
PECT OF RIGHT 
Rapivus. 
fe, Articulatory 
surface for capi- 
tellum of hu- 
merus; hd, head 
—the point of 
the arrow is on 
the articulating 
surface for the 
lesser sigmoid 
cavity; nk, neck; 
sc, articulatory 
surface for 
scapholunar;_ st, 
styloid process; 
tb, tubercle; u/, 
facet for ulna. 
Equus scarcely 
(Fig. 37). 
The bones of the antebrachium in 
many mammals are more or less coal- 
esced. In the Chiroptera and many of the 
Ungulates the radius is enlarged at the 
expense of the ulna, whose proximal third 
The primitive Ungulates 
of the lower tertiary period possessed a 
complete ulna as well as radius.. The 
phylogeny of the horse’s limb illustrates 
the gradual development of the ante- 
brachium of the Equide (Fig. 35). 
Fossil remains reveal the fact that mam- 
mals existed as early as the triassic 
period, when the sedimentary rock form- 
ing the triassic strata was laid down. 
This probably occurred 10,000,000 years 
ago. 
According to paleontological investi- 
gations, the Ungulata arose from the 
Condylarthra, a group of small five-toed 
mammals of the lower Eocene, best repre- 
sented by the typical genus Phenacodus. 
In this genus and its successor, Hyraco- 
therium, the ulna and radius are well 
developed and distinct. Orolippus, the 
descendant of Hyracotherium, also shows 
a distinct radius and ulna, but in the later 
forms of the horse line the ulna gradually 
diminishes in size and becomes more and 
only remains. 
more coalesced with the radius, until in 
more than the proximal third remains 
