3-28 



ANATOMY OF VERTEBRATES. 



210 



as in some Keptiles, one for the fourth and the other for the fifth 

 metatarsals. In both Monotremes there is a sesamoid bone, fig. 

 209, *, placed at the interspace between the astragalus and the 

 naviculare; a second supernumerary bone, ib. **, is articulated 

 to the posterior part of the astragalus, and supports the per- 

 forated spur Avhich characterises the male, sex, fig. 199, K, d, e. 

 The calcaneum of the Ornithorhynchus, cl, terminates by sending 

 outward a short obtuse tuberosity ; in the Echidna this part is 

 more slender, and is singularly directed inward and forward 

 nearly in a line with the digits, fig. 210, c. 



The astragalus in the Ornithorhynchus presents a double 

 trochlea above for the tibia and fibula, and a depression on its 

 inner side, which receives the incurved mal- 

 leolus of the tibia, almost as in the Sloths. 

 The toes have the same number of bones 

 as in other Mammals ; their size and form 

 are more alike in the two Monotrematous 

 genera than those of the fingers : the un- 

 gual phalanges, like the claws they support, 

 are more curved than those on the fore-foot, 

 but like them they are perforated on their 

 inner and concave side, fig. 210. 



§180. Skeleton of Mar supialia. — A. Ver- 

 tehral Column. — The number of ' true ' ver- 

 tebra3 is the same in all the Marsiqnalia, 

 viz. 20 ; that of the dorsal and lumbar series 

 varying according to the number of long 

 and free ribs, e. g. c? 12 Z 7 in Petaurists, 

 ^15 Z 4 in Wombats, ^13 Z in other genera ; 

 the cervicals are seven in all. 

 In the Koala the length of the spine of the first dorsal hardly 

 exceeds that of the last cervical, but in all other Marsupials the 

 difference is considerable, the first dorsal spine being much longer ; 

 those of the remaining dorsal vertebras progressively diminish in 

 length and increase in breadth and thickness. They slope toward 

 the centre of motion, which is shown by the verticality of its 

 spine : this, in the Perameles, is at the eleventh dorsal vertebra, 

 in Potoroos and Kangaroos, fig. 211, at the ninth -twelfth, in the 

 Petaurists at the thirteenth vertebra. In the Phalangers, Koala, 

 and Wombat, the flexibility of the spine is much diminished, and 

 the centre of motion is not defined by the convergence of the 

 spinous process toward a single vertebra, but they all incline 

 slightly backward, fig. 212. 



Bones of hind-foot, plantar 

 aspect, Echidna setosa. 



