145 



line of separation be manifest externally. In a second species of each 

 of the above genera it is in close contact with the coiTesponding; 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 louver half, so as to be 

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

 tact. In each of these genera therefore, in which locomotion is 

 principally performed by the hinder extremities, fixity and strength 

 is gained by the structure of the bones of the leg. In the other 

 genera, as Phascolarctos, PJiascolomys, Phalangista, Petmrvs, Didel- 

 phis, and Dasyitrus, the tibia and fibula are so connected together, 

 and with the tarsus, that the foot enjoys a movement of rotation ana- 

 logous to the pronation and supination of the hand ; and in the Pe- 

 taurists, Phalangers, Opossums, and Koala, the inner toe is so placed 

 and organized as to perform the office of an opposable thumb, whence 

 these Marsupiata have been termed pedimana or foot-handed. It 

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

 relate. In the Wombat, Koala, Petaurists, and Phalangers it ex- 

 pands 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 shghtly convex, while that of the tibia is slightly 

 concave. The proximal extremity of the fibula is also much en- 

 larged, but compressed, obliquely truncated, and giving oflF two tube- 

 rosities from its exterior surface ; to the superior of these a large 

 sesamoid bone is articulated ; we observe the same sesamoid at- 

 tached to the upper end of the fibula in a Dasyurus macrurus. Tem- 

 minck figures it in the Phalangista ursina and Didelphis Philander. 



" This enlarged and elevated proximal end of the fibula, with its 

 superimposed sesamoid, obviously represents the olecranon of the 

 ulna, and beautifully illustrates and establishes the analogies long 

 ago pointed out between the radius and tibia, the ulna and fibula, 

 by my revered preceptor in anatomy. Dr. Barclay*. 



' " I find the following 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 internal surface for the inner malleolus flat- 

 tened, and at light angles with the preceding. But the outer 

 articular surface presents a triangular flattened form ; and instead of 

 being bent down parallel with the inner articulate surface, slopes 

 awav 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 ver}^ small proportion of the outer part of the inferior surface of 



* See his admirable ' Description of the Arteries of the Human Body,' 

 pp. 2.58, 259, and his ' Explanations of Mitchell's Engravings of the Bones, 

 4to., Edin. 1824, Expl. of Tl. xxiv.' Both Dr. Barclay's analogies of the 

 bones of the adantal and sacral exti-emities, and my hypothesis of the nature 

 of the marsupial bones, have been reproduced in the' past year as novel dis- 

 coveries, by two French anatomists ; the one by Dr. Flourens in an interest- 

 ing and ingenious paper in the ' .\nnales des Sciences Nat., Oct. 1838,' the 

 other by M. Gervais in the ' Zoologie de la Favorite,' Partie III. p. 100. 



