XIX.] MARSUPIALIA. 323 



adapted for plantigrade progression on the ground. It does 

 not even present any modifications corresponding to those 

 observed in the manus of the same animals. The normal 

 number of tarsal bones, and the complete number of pha- 

 langes, are always present in each digit. Of these the 

 second and third are usually the longest, the fourth next, 

 and the first and fifth shortest. 



The little prehensile-tailed Tree Anteater {Cydothurus 

 didadyliis) has the pes modified into a climbing organ. 

 The hallux is rudimentary, consisting of a metatarsal and 

 one phalanx, concealed beneath the skin, but the four other 

 toes are subequal and much curved, with long, pointed, 

 compressed, ungual phalanges. The tuber calcauei is di- 

 rected towards the plantar surface, and parallel with it, and 

 extending to about double its length, is the greatly elongated 

 sesamoid ossicle of the tibial side of the foot. These 

 together support the prominent calcarine cushion to which 

 the nails are opposed in climbing. 



In the Marsupialia, the hind foot is subject to great 

 modifications, some of the genera presenting very striking 

 deviations from the typical condition. 



The seven bones usually found in the Mammalian tarsus 

 are always present and distinct from each other, but the 

 astragalus is relatively smaller and more flattened than in 

 Placental Mammals. In the climbing Marsupials espe- 

 cially, the articular surface for the fibula, instead of being 

 perpendicular to that for the lower end of the tibia, is 

 almost in the same plane with it ; and in all, the " head," or 

 portion of the bone which projects forwards to articulate 

 with the navicular, is very slightly developed. 



Ill the American Opossums {Didelpiiidce), the foot is 

 short and broad ; the hallux is stout, placed at right angles 

 with, and opposable to, the other four digits ; it has a short, 



Y 2 



