362 ANATOMY OF VERTEBRATES. 



zium, trajjezoides, and os magnum. The entocuneiform bone is the 

 largest of the three in the Wombat^ although it supports the 

 smallest of the toes. It is of course more developed in the Pedi- 

 manous Marsupials, where it supports a large and opposable thumb. 



In the Wombat the metatarsals progressively increase in length 

 and breadth from the innermost to the fourth ; the fifth or outer- 

 most metatarsal is somewhat shorter but twice as thick, and it 

 sends oiF a strong obtuse process from the outside of its proximal 

 end. A corresponding process exists in the Phalangers, fig. 228. 

 The innermost metatarsal of the Wombat, fig. 212, i, supports only 

 a single phalanx ; the rest are succeeded by three phalanges each, 

 progressively increasing in thickness to the outermost ; the ungual 

 phalanges are elongated, gently curved downward, and gradually 

 diminish to a point. 



In the Myrmecobius the tibial or innermost toe is represented 

 by a short rudimental metatarsal bone concealed under the skin. 

 In the Dasyures the innermost toe has two phalanges, but it is the 

 most slender and does not exceed in length the metatarsal bone of 

 the second toe. In the Petaurists it is rather shorter than the 

 other digits, but is the strongest, and in Petaurus taguanoides the 

 terminal phalanx is flattened and expanded ; the toes are set wide 

 apart in this genus. In the Opossums and Phalangers the inner- 

 most metatarsal bone is directed inward apart from the rest, and 

 together with the first phalanx is broad and flat. The second 

 phalanx in the Opossums supports a claw, but in the Phalangers 

 is short, transverse, unarmed, singularly expanded in Ph. CooMi, 

 but almost obsolete in Ph. ursina (fig. 228, i). In all the preced- 

 ing genera there are two small sesamoid bones on the under side 

 of the joints of the toes, both in the fore and hind feet. 



The commencement of a degeneration of the foot which is 

 peculiar to Marsupial animals may be discerned in the Petaurists, 

 in the slender condition of the second and third toes, as compared 

 with the fourth and fifth. In the Phalangers this diminution of 

 size of the second and third toes, counting from the hallux, is 

 more marked. They 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 ungual phalanges, whence the name of 

 Phalangista applied to this genus (fig. 228, 2 and s). 



In the saltatorial genera of Marsupials the degradation of the 

 corresponding toes is extreme ; but though reduced to almost fila- 

 mentary slenderness they retain the usual number of phalanges, 

 and the terminal one of each is armed \Ai\\ a claw. These claws 

 being the only parts of the rudimental digits which project freely 



