MARSUPIALIA. 



285 



fhejlbula is complete, and forms the ex- 

 ternal malleolus in all the Marsupials. In one 

 species of Hypsiprymnus and in one species of 

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

 the lower part of the tibia, though the line of 

 separation be manifest externally. In a second 

 species of each of the above genera it is in close 

 contact with 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 contact and attachment. 

 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 ensure a due 

 degree of fixity and strength ; while in the 

 other marsupial genera, as Pfiascolarctos, Phas- 

 colomys, Phalangista, Pttaurus, Didelphis, 

 and I)asyurus, the tibia and fibula are so loosely 

 connected together and with the tarsus, that 

 the foot enjoys a movement of rotation analo- 

 gous 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 organized as to perform the office 

 of an opposable thumb, whence these Marsu- 

 pials have been termed Pedimuna or foot- 

 handed (fig. 111). 



It is to this prehensile power that the modi- 

 fications of the fibula chiefly relate. In the 

 Wombat, Koala, Petaurists, and Phalangers 

 it expands to nearly an equal size with the tibia 

 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 truncated, and 

 giving off two tuberosities from its exterior 

 surface ; to the superior of these a large sesa- 

 moid bone (c, fig. Ill) is articulated; I have 

 observed the same sesamoid attached to the 

 upper end of the fibula in a Dusyurus macru- 

 rus and Petaurus taguanoides. M. Temminck 

 figures it in the Didelphys ursina and Didel- 

 phys Philander. This sesamoid and the ex- 

 panded process to which it is attached form the 

 analogue of the olecranon ; and the corres- 

 pondence of the fibula with the ulna is very 

 remarkably maintained in the Pet. taguanoides, 

 in which the proximal articular surface of the 

 fibula is divided into two facets, one playing 

 upon the outer condyle of the femur, the other 

 concave, vertical, and receiving an adapted con- 

 vexity on the outer side of the head of the tibia, 

 which rotates thereupon exactly like the radius 

 in the lesser sigmoid cavity of the ulna. 



In the scansorial and gradatorial Marsupials 

 the bones of the hinder and fore extremities 

 are of nearly equal length, but in the saltatory 

 species the disproportion in the development of 

 the bones of the hind leg is very great, especi- 

 ally in the Kangaroos and Potoroos (fig. 103). 

 However, in those singular species of Hi/psi- 

 prymnus which inhabit New Guinea and take 



Fig. 111. 



2 3 



Bones of the leg and foot, Phalangista. 



refuge in trees, the organization of the Kangaroo 

 is modified and adapted so as to make climb- 

 ing a possible and easy action. The fore and 

 hind legs are here more equally developed, 

 and the claws on the two larger toes of the 

 hind feet are curved instead of straight. In a 

 skeleton of one of these scansorial Potoroos, 

 the Hypsiprymnus ursinus, in the Museum at 

 Leyden, in which the humerus is three inches 

 and a half long, the femur does not quite equal 

 five inches in length : the ulna is nearly four 

 inches, the fibula nearly five inches in length. 

 The fibula is also less firmly connected with 

 the tibia than in the great Kangaroo. 



The following is the structure of the tarsus 

 in the Wombat. The astragalus is connected 

 as usual with the tibia, fibula, calcaneum, and 

 scaphoides. The upper articular surface for 

 the tibia is as usual concavo-convex, the inter- 

 nal surface for the inner malleolus flattened and 

 at right angles with the preceding, but the 

 outer articular surface presents a triangular 

 flattened form, and instead of being bent down 

 parallel with the inner articular surface, slopes 

 away at a very open angle from the upper 

 surface, and receives the articular surface of 

 the fibula so as to sustain its vertical pressure. 

 A very small proportion of the outer part of 

 the inferior surface of the astragalus rests upon 



