SKELETON OF MARSUPIALIA. 359 



and Potoroos the anterior part of the head is much produced, 

 and in the young animal its ossification commences by a centre 

 distinct from the ordinary proximal epiphysis of the bone. A 

 strong ridge is continued down from this protuberance for about 

 one-sixth the length of the tibia. In the Koala a strong tube- 

 rosity projects from the anterior part of the tibia at the junction 

 of the upper with the middle third. In this species and in the 

 Wombat, as also in the Opossums, Dasyures, Phalangers, and 

 Petaurists, the shaft of the tibia is somewhat compressed and 

 twisted; but in the Kangaroos, Potoroos, and Perameles the 

 tibia is prismatic above and sub-cylindrical below. The internal 

 malleolus is very slightly produced in any Marsupial, but most so 

 in the Wombat. 



The^/ibida, ib. 67, is complete, and forms the external malleolus 

 in all the Marsupials. In one species of Hijpsijjrijmnus and in one 

 species of Perameles (P. lagotis) it is firmly united to the lower 

 part of the tibia, though the line of separation be manifest exter- 

 nally. In a second species of each of the above genera it is in 

 close contact wdth the corresponding part of the tibia, but can be 

 easily separated from that bone. In the Great Kangaroo the 

 fibula is also a distinct bone throughout, but it is remarkably 

 thinned and concave at its lower half, so as to be adapted 

 to the convexity of the tibia, with which it is in close attach- 

 ment. In each of these genera, therefore, in which locomotion 

 is principally performed by the hinder extremities, we perceive 

 that their osseous structure is so modified as to insure a due de- 

 gree of fixity and strength ; while in the other Marsupial genera, 

 as Phascolarctos, Phascolomys, Plialangista, Petaurus, Didel2)hys, 

 and Dasyurus, the tibia and fibula are so loosely connected toge- 

 ther and with the tarsus, that the foot enjoys a movement of rota- 

 tion analogous to the pronation and supination of the hand. This 

 property is especially advantageous in the Petaurists, Phalangers, 

 Opossums, and Koala, because in these the inner toe is so placed 

 and organised as to perform the oflfice of an opposable thumb, 

 whence these Marsupials have been termed Pedimana, or foot- 

 handed (fig. 228). 



It is to this prehensile power that the modifications of the fibula 

 chiefly relate. In the Wombat, fig. 212, 67, Koala, Petaurists, and 

 Phalangers, it expands to nearly an equal size with the tibia, 66, at 

 the distal extremity, and takes a large share in the formation of the 

 tarsal joint ; but the articular surface is slightly convex, while 

 that of the tibia is slightly concave. The proximal extremity of 

 the fibula is also much enlarged, but compressed and obliquely 



