LIMBS 



171 



(vtrissodactyle form), 1 and eventually is the only complete one 

 remaining, in cloven-footed Ungulates the third and fourth digits 



FIG. 134. FORE -FOOT OF ANCESTRAL FORMS OF THE HORSE. 1. OROHIPPUS 

 (Eocene). 2. MESOHIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene). 

 4. PROTOHIPPUS (Upper Pliocene). 5. PLIOHIPPUS (Uppermost Pliocene). 

 6. EQUUS. 



are both functional and equally strongly developed (artiodactyle 



form, Fig. 135) ; their metacarpals may be united with one 



another and with the vestiges of 



the proximal ends of the second 



and fifth to form a " cannon- 



bone," while the other digits are 



gradually reduced. A similar re- 



duction takes place in the hind- 



foot, and is here as a rule more 



rapid. 



The Protungulata must origin- 

 ally have been pentadactyle and 

 plantigrade (i.e. the whole foot 

 rested on the ground) or semi- 

 plantigrade, with ungual phal- 

 anges but little broadened. On 

 the gradual elongation and 

 straightening out of the limbs 

 and unequal development of the 

 digits, they become digitigrade 

 (as in most Carnivora), and 

 eventually unguli grade, only the 

 hoofs at the extremity of the 

 distal phalanges bearing the 

 weight of the body. 



The Tylopoda, as well as Ele- 

 phants (Subungulata) have not FIG. ~ 135. -SKELETON OF THE 'LEFT 

 reached the unguligrade stage: FORE-LIMB OF A, PIG; B, HYO- 

 they are practically digitigrade, a MOSCHUS ; C, TRAGULUS ; D, ROE- 

 large integumentary pad or sole 

 (cf. Fig. 24), from which the small 

 " hoofs" project, bearing the main weight of the body (Fig. 136). 



Some of the many other adaptive modifications of the limbs in 



1 The Tapir has four digits on the fore-foot and three on the hind-foot ; the 

 Rhinoceros ha,s three on each foot, 



^Girrod.) G 



