Nos. 455-456.] 



MAMMALIA. 



great relative len^^lh of the humerus and femur is a striking- 

 feature in the hnibs of all early Tertiary mammals. In the 

 evolution of the limbs for running the femur and humerus have 

 generally been reduced in length and c()m[)acted into the tlank. 

 while the lower legs and feet are elongated. This i)laces the 

 heavy muscles high up on the limb, and by bringing its centre 

 of gravity near to the proximal joint, enables it to swing more 

 rapidly through a considerable arc. Thus the animal gains in 

 length of stride without losing in quickness of step and can 

 maintain a high s])eed for a long distance with less fatigue. 



8. — i Ina and radius separate, equal in size, radius leith 

 round head, f^erniittino- free supination and pronation. Tibia and 

 fibula separate zeitli probably more limited motion. 



9. -- Wnst and ankle very Jlexible, all the earpals separate and 

 a eentrale present. Astragalus leitli plat trochlea, distinct neck 

 and rounded head. This type of carpus and tarsus is found in 

 all Hasal Eocene mammals. It is retained with but little change 

 in primates, insectivores, most rodents, and some other groups. 

 Its gradual conversion into the various types seen in other 

 groups is demonstrable in the Amblypoda, Condylarthra, Car- 

 nivora, partly so in the Proboscidea and Edentata, but not in 

 the Artiodactyla and Perissodactyla which have already devel- 

 oped their peculiar types of astragali when they first appear m 

 the geological record. 



10. — Five digits on each foot, the joints permitting of very 

 free motion of fingers and toes, which were tipped by small 

 clazvs. The argument for the derivation f W als from 

 pentadactyl ancestors has been fully set forth by Cope and 

 others. The derivation of the hoofed from clawed types is like- 

 wise indicated in various hnes of descent as now^ known. 



11. — First digit more or less opposable in both manus and pes. 

 This is contrary to the usual assumption that the opposable 

 thumb found in several groups of arboreal mammals is in each 

 case a new adaptation to their habits of life. But there is con- 

 siderable evidence for it. In the first place, as far as we can 

 trace back the history of each of the arboreal groups, we find 

 their first ancestors with the first digit as fully opposable as m 

 the modern representatives {e.g., the Middle Eocene primate 



