No. 445-] 



HABITS IN MAMMALS. 



7 



more robust. The metapodials arc shorter and stouter than in 

 the horse, those of digits II and W particularly being much 

 more prominent. 



Thoatherium from the same beds is nionodact) 1, the lateral 

 metapodials being even more vestigial than in Iu[uus which it 

 parallels, and as in Proterotherium, the phalanges, especially the 

 proximal and ungal of the remaining digit are much more 

 slender than in the horse, the ungal being cleft. A curious 

 admixture of perissodactyl and artiodactyl characters is seen in 

 the feet of the Litopterna for they have the odd toed feet of the 

 Perissodactyls together with the characteristic double tarsal 

 joint, though not to so great an extent, of the artiodactyls. 



The Artiodactyla early lose the hallux and pollex, for except in 

 Oreodon and Agriochoerus we have no instance of their survival 

 and while digits III and IV are equally well developed, II and 

 V suffer all degrees of reduction from that seen in the swine to 

 the total disappearance in the camel and Antilocapra. 



The swine are four toed, the lateral digits being sub-functional. 

 Dicotyles, the peccary shows an advance over most Suidae in 

 that digit V of the pes is entirely wanting giving an asym- 

 metrical foot, of uncommon occurrence in the order though 

 found in the Anoplotheres as well. In Dicotyles the metacar- 

 pals are slightly fused at their proximal end while in the 

 metatarsals the fusion extends over half the length of the bones. 

 The Pleistocene genus Platygonus shows a still greater special- 

 ization as it is structurally didactyl, but a splint of the fifth 

 metatarsal remaining. The metapodial bones show a greater 

 degree of fusion than in Dicotyles. 



The Tragulidae or chevrotains are in a sense transitional 

 between the swine and the true deer for, while four toed, the 

 lateral toes are functionless although in the existing genera 

 Tragulus and Dorcatherium (Hyomoschus) the lateral meta- 

 podials are entire. Fusion of the median metatarsals to form a 

 canon bone is found in Tragulus, but not in Dorcatherium which 

 together with its somewhat better developed lateral digits pre- 

 sents a more generalized condition than does Tragulus. Gelo- 

 cus, an extinct form ranging from the Kocene to the Oligocene, 

 is more specialized than either of the existing genera in that the 



