70 CAUDAL VERTEBRA. [chap. 



In the Marsupialia, as might be supposed in so hetero- 

 geneous a group, there is great diversity in the condition of 

 the caudal vertebrae. 



In the Wombat (Phascolomys) and Koala (Phascolardos) 

 "the tail is comparatively rudimentary. 



In the Kangaroo, on the other hand, it is very large, and 

 serves as an organ of support when standing upright. It is 

 composed of from 21 to 25 vertebrae, the first few with short 

 bodies and large processes ; afterwards the bodies lengthen 

 out, becoming cylinders contracted in the middle. The 

 zygapophyses soon cease, but the metapophyses continue 

 longer. The neural arch is not continued longer than about 

 the middle of the tail ; the transverse processes are gradually 

 placed further and further back on the vertebra, and then a 

 new one arises near the anterior end, so that they become 

 double. 



The chevron bones are placed quite between the verte- 

 brae, so that it is difficult to say to which they most 

 properly belong. In the proximal part of the tail their 

 free edge is compressed and develops a process forwards 

 and backwards, giving a hatchet shape when seen side- 

 ways. Further back they also send out broad processes 

 laterally, so as to be cruciform, with a flat inferior 

 surface. 



In some Marsupials the tail is prehensile, as in the 

 Opossums (Didelfthys), with from 19 to 35 vertebrae, and the 

 Phalangers (Phalangista), with from 21 to 31. 



Chevron bones are generally present in the tails of all 

 the Marsupials, except the Wombat and Koala. In the 

 Thylacine they are few, and comparatively rudimentary. 



The tails of the two animals composing the order 

 Monotremata differ considerably. 



The Echidna has 12 caudal vertebrae. These have no 



