ELEMENTS OF MAMMALIAN ANATOMY. 



capitellum of the humerus. The styloid process projects 

 from the distal end parallel with the process of the same 

 name on the ulna. 



The bones of the antebrachium in 



r 



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 

 only remains. 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 Equidse (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 palaeontological investi- 



FIG. 34. MEDIAL 

 OR INNER AS- 

 PECT OF RIGHT gations, the Ungulata arose from the 

 RADIUS. ~ 



t ondylarthra , a group of small five-toed 



mammals of the lower Eocene, best repre- 



fc, Articulatory 

 surface for capi- 

 tellum of hu- 

 merus; hd, head sented by the typical genus Phenacodus. 



In this genus and its successor, Hyraco- 

 theriwn, the ulna and radius are well 

 developed and distinct. Orohippus, 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 

 more coalesced with the radius, until in 



Equus scarcely more than the proximal third remains 



(Fig. 37)- 



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; ul, 

 facet for ulna. 



