326 THE ANATOMY OF VEETEBRATED ANIMALS. 



always nineteen ; and, of these, six are usually dorsal. The 

 atlas is generally incompletely ossified in the ventral 

 median line. The manus usually possesses five digits, but 

 in Peraineles and Chceropus the outer digits become rudi- 

 mentary. 



The fibula is always complete at its distal end. In some 

 cases it becomes ankylosed with the tibia, while in the 

 Wombat (Phascolomys), the Phalangers (Phalangistidce), and 

 the Opossums {Didelphidce), it is not only free, but is capable 

 of a rotatory movement upon the tibia, similar to the move- 

 ment of pronation and supination of the radius upon the 

 ulna in Man. The rotation of the fibiila towards the ventral 

 side of the tibia is effected by a muscle which, in great 

 measure, occupies the place of the interosseous ligament, 

 and is analogous to the pronator quadratus in the fore limb. 

 This muscle is antagonised by the extensors of the digits, 

 so far as they arise from the fibula. 



The digits of the pes vary remarkably in their form and 

 relative development among the Marsupialia ; the different 

 subdivisions of the order being very well distinguished by 

 the modifications of the hind foot. 



Thus in the especially carnivorous marsupials— the Didel- 

 phidce, of America, and the Dasyuridce, of the Australian 

 province— the second and third digits of the pes are not 

 Tinited together by the integument. In the Didelphidce, the 

 hallux is nail-less, but large and opposable, so as to convert 

 the pes into a prehensile organ like that of many Primates ; 

 in the Dasyuridce, on the other hand, the hallux is rudi- 

 mentary or absent. In all the other marsupials, the second 

 and third digits of the pes are syndactyle, or united together 

 by integument. In the Wombat, the fourth toe is bound 

 together with the other two, and the small hallux is devoid 

 of a nail. In the Phalangers, only the second and third 

 toes are syndactyle, and they are slender, compared with 

 the other digits, while the hallux is well developed and 

 opposable. In the Peramelidce (Bandicoots) and Macro- 

 "podidoB (Kangaroos), the metatarsus is much elongated, and 

 the second and third digits united and slender, while the 



