45^ Zoological Society. 



" In the great Kangaroo the fibula is a distinct bone throughoat, 

 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. 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, Phascolomys, Phalangista, Petaurus, Didel- 

 phis, and Dasyurus, 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 slightly convex, while that of the tibia is slightly 

 concave. 



** The analogy of the carpal and tarsal bones is very clearly illus- 

 trated in the Wombat. The anchylosed naviculare and lunare of the 

 hand correspond with the astragalus and naviculare of the foot, 

 transferring the pressure of the focile majus upon the three inner- 

 most bones of the second series. The long backward projecting pisi- 

 form bone of the wrist closely resembles the posterior process of the 

 i)S calcis ; the articular portion or body of the os calcis corresponds 

 •with the cuneiform ; the large unciform represents the cuboides, and 

 performs the same function, supporting the two outer digits ; the 

 three cuneiform bones are obviously analogous to the trapezium, tra- 

 pezoides, and os magnum. 



" The commencement of a degeneration of the foot, which is pecu- 

 liar to, and highly characteristic of, the Marsupial animals, may be 

 discerned in the Petaurists, in the slender condition of the second 

 and third toes, as compared with the other three. In the Phalangers, 

 this diminution of size of the second and third toes, counting from 

 the thumb, is more marked. Thej are also both of the same length, 

 and have no individual motion, being united together in the same 

 sheath of integument as far as the ungueal phalanges, whence the 

 name of Phalangista applied to this genus. In the saltatorial genera 

 of Marsupiata the degradation of the corresponding toes is extreme ; 

 but though reduced to almost filamentary slenderness, they retain 

 the usual number of phalanges, the terminal ones being armed with 



