MARSUPIALIA. 
The fibula is complete, and forms the ex- 
ternal malleolus in all the Marsupials. In one 
species of Hypsiprymnus and in one species of 
erameles (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 moditied as to ensure a due 
degree of fixity and strength; while in the 
other marsupial genera, as Phascolarctos, Phas- 
colomys, 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- 
= have been termed Pedimana or foot- 
anded (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. 111) is articulated; I have 
observed the same sesamoid attached to the 
upper end of the fibula in a Dasyurus 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 Huypsi- 
prymnus which inhabit New Guinea and take 
285 
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. Ina 
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, caleaneum, 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 
