THE SKELETON IN MAMMALIA. THE PES. 521 



THE PES. 



The skeleton of the pes is in most respects a counterpart 

 of that of the manus. Just as in the manus if one digit is 

 absent it is the pollex, so in the pes it is the hallux. But 

 while in the manus the third digit is always well developed, 

 however much the limb may be modified, in the pes any of the 

 digits may be lost. In all mammals the tibiale and inter- 

 medium fuse to form the astragalus, and the fourth and fifth 

 tarsalia to form the cuboid. Sesamoid bones are considerably 

 developed. In almost every case the phalanges and first meta- 

 tarsal have epiphyses only on their proximal ends, while the 

 remaining four metatarsals have epiphyses only on their distal 

 ends. 



In the MONOTREMATA all the usual tarsal bones are distinct, 

 and the five digits have the normal number of phalanges. 

 Several sesamoid bones are developed, the most important one, 

 found only in the male, being articulated to the tibia and bearing 

 the curious horny spur. The ungual phalanges of the pes like 

 those of the manus, are deeply cleft at their extremities. In 

 the Echidnidae the pes is turned outwards and backwards in 

 walking. 



In the MARSUPIALIA the pes is subject to great modifica- 

 tions, but in every case the seven usual tarsal bones are dis- 

 tinct. In the Didelphyidae the foot is broad, all five digits are 

 well developed, and the hallux is opposable to the others. 

 In the Dasyuridae the foot is narrow, and the hallux may be 

 very small, or as in Thylacinus completely absent. In Noto- 

 ryctes the pes is much less abnormal than the manus, and 

 all five digits have the usual number of phalanges. The fifth 

 metatarsal has a curious projecting process, and there is a 

 large sesamoid above the hallux. In the Wombats (Phascolo- 

 myidae) the foot is short and broad, the digits are all distinct, 

 arid the hallux is divaricated from the others. 



In the remaining marsupials the second and third meta- 

 carpals and digits are very slender, and are enclosed within a 



