THE DIDELPHIA. 270 



equal number of incisors in each jaw, the number of the 

 upper being usually in excess of that of the lower jaw. 



The number of the dorso-lumbar vertebrae is almost 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 Perameles and 

 Ghcerojnis the outer digits become rudimentary. 



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

 cases it becomes anchylosed with the tibia, while in the 

 Wombat (PhascoIo77iys), the Phalangers [Phalangistidm)^ 

 and the Opossums {DidelpliicB)^ it is not only free, but is ca- 

 pable of a rotatory movement upon the tibia, similar to the 

 movement of pronation and supination of the radius upon the 

 ulna in Man. The rotation of the fibula toward the ventral 

 side of the tibia is efi'ected by a muscle which, in great meas- 

 ure, occupies the place of the interosseous ligament, and is 

 analogous to the pronator quadratus in the fore-limb. This 

 muscle is antagonized 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 Marsiipialia; 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- 

 phidoe^ of America, and the Dasyuridm, of the Australian 

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

 united together by the integument. In the Didelphidm^ the 

 hallux is nailless, 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 rudimen- 

 tary or absent. In all the other marsupials, the second and 

 third digits of the pes are syndactyle, or united together by in- 

 tegument. In the Wombat, the fourth toe is bound together 

 with the otlier two, and the small hallux is devoid of a nail. 

 In the Phalangers, only the second and third toes are syn- 

 dactyle, and they are slender, compared with the other digits, 

 while the hallux is well developed and opposable. In the 

 PeramelidcG (Bandicoots) and Macropodldce (Kangaroos), the 

 metatarsus is much elono;ated, and the second and third di2:its 

 united and slender, while the fourth toe is very large. The 

 hallux is reduced to its metatarsal bone in the Peramelidc^^ 

 and the fifth digit is small or rudimentary. In the Kangaroos, 

 the hallux disappears altogether, but the fifth digit remains 

 well developed, though not so large as the fourth. 



