The fibula is complete, and forms the ex ternal malleolus in all the Marsupials. In one species of Hypsiprynatus 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 Phascolaretos, Phas eolomys, Phalangista, Petaurus, Didelphis, and Dasyurus, 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 Pedimana 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 (e, fig. 111) is articulated; I have observed the same sesamoid attached to the upper end of the fibula in a Dasyurns 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 Hypsi prymnus which inhabit New Guinea and take 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 the calcaneum ; a greater part of the super incumbent pressure is transmitted by a trans versely extended convex anterior surface to the scaphoid and cuboid bones. This form of the astragalus is also characteristic of the Koala, Petaurists, Dasyures, and the Pedimanous Marsupials ( d, fig. 111). In the Kangaroos, Potoroos, and Perameles which have the pcdes saltatorii, the fibular articular surface of the astragalus is bent down as usual at nearly right angles with the upper tibial surface.
The calcancanz in the Wombat presents a ridge on the outer surface which serves to sus tain the pressure of the external malleolus which is not articulated to the side of the astragalus. The internal surface which joins the astraga lus is continuous with the anterior slightly concave surface which articulates with the cuboides. The posterior part of the bone is compressed, it projects backwards for nearly an inch, and is slightly bent downwards and inwards. This part is relatively shorter in the Koala, Phalangers, Opossums, and Petaurists, but it is as strongly developed in the Dasynri as in the Wombat. The anterior part of the calcaneum of the Phalangers is shown at e, fig. 111.