360 



ANATOMY OF VERTEBRATES. 



truncated, and giving off two tuberosities from its exterior surface ; 

 to the superior of these a large sesamoid bone, fig. 228, c?', is ar- 

 ticulated ; a similar sesamoid ' fabella ' is attached to the upper end 

 of the fibula in a Dasyurus macrurus and Petaurus taguanoides. 

 M. Temminck figures it in the Didelphys ursina and Didelphys 



Philander. This sesamoid and the ex- 

 panded process to which it is attached 

 form the homotype of the olecranon, 

 fig. 212, 55; and the correspondence 

 of the fibula with the ulna is very 

 remarkably maintained in the Pet. 

 taguanoides, in which the proximal 

 articular surface of the fibula is divi- 

 ded into two facets, one playing upon 

 the outer condyle of the femur, the 

 other concave, vertical, and receiving 

 an adapted convexity on the outer 

 side of the head of the tibia, which 

 rotates thereupon 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 de- 

 velopement of the bones of the hind 

 leg is very great, especially in the 

 Kangaroos and Potoroos, fig. 211. 

 However, in those singular species 

 of Hypsiprymnus which inhabit New 

 Guinea and take refuge in trees, the 

 organisation of the Kangaroo is mo- 

 dified and adapted so as to make 

 climbing 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, a, is connected as usual 



Bones of the leg and foot, Phalcmgista. 



and Phalanger, 



fig. 228. 



