No. 445.] 



HABITS IN MAMMALS. 



5 



though the pollex is reduced, while in the foot but three digits 

 are functional, the lateral digits being vestigial. 



In Dasyprocta, the agouti, however, the hand remains dis- 

 tinctly pentadactyl though digits I and V are subfunctional, while 

 in the foot the digits are reduced to three. Thus the foot is 

 both structurally and functionally tridactyl, the metatarsals being 

 closely pressed together though not fused. The compact pes 

 thus formed is tending toward that possessed by the Dipodid?c 

 {vide infra). 



The Caviidae, represented by the Patagonian cavy, Dolichotus, 

 show a further reduction over Dasyprocta in that in the former 

 the manus is tetradactyl while the pes is in the same stage of 

 reduction in each. 



Among the truly saltatorial rodents a range of specialization 

 is shown, starting from Perodipus, in which the manus and pes 

 are each pentadactyl though there is considerable disparity of 

 size between the fore and hind limbs, the progression being 

 by leaping with the hind feet. Dipodomys, the kangaroo rat, 

 belonging, together with Perodipus, to the American family 

 Heteromyidse, has much the same proportions, but the pes is 

 tetradactyl. In Pedetes, the African jumping hare, the pes is 

 tridactyl, the median digit being much the longest ; while in the 

 true jerboas, as Dipus, the disparity between the limbs reaches 

 its greatest development and the elongate metatarsals are fused 

 into one very bird-like bone. The digital formula is manus 5, 

 pes 3 ; but the clawless pollex is evidently undergoing reduction. 



Among the ungulates I know of none in which cursorial 

 adaptation is manifest, which have not already functionally lost 

 the pollex and hallux, that is with the exce])lion of I'henacodus 

 among the condylarths which was [)entadactyl and undoubtedl)' 

 could run though exhibiting no very marked c ursorial adaptation. 

 In general, while the pes is often more specialized than the 

 manus there is far more uniformity in the plan of modification 

 of fore and hind feet than was observed in the rodents and 

 diprotodont marsupials. The artiodactyl and penssodactyl 

 stems have modified the feet in such different ways that it 

 becomes necessary to give each group separate treatment. 



Among the Perissodactyla, the rinocerotine group, mostly of 



