76 CAUDAL VERTEBRA. [CHAP. 



processes increasing in size instead of gradually diminishing, 

 as is almost universally the case. 



The chevron bones are usually much developed. They 

 are Y-shaped, having long, simple, compressed spines in 

 Orycteropus ; V-shaped in Manis and most Armadillos ; but 

 in Priodon (Fig. 30) they have wide, diverging, lateral pro- 

 cesses, instead of a median spine. They are attached 

 rather to the vertebra in front than to that behind them. 



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

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

 the caudal vertebras. 



In the Wombat (Phascolomys) and Koala (Phascolarctos) 

 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 2 1 to 25 vertebras, the first few with short bodies 

 and large processes ; afterwards the bodies lengthen out, be- 

 coming 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 anew 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 (Didelphys), with from 19 to 35 vertebras, and the 

 Phalangers (Phalangista) with from 21 to 31. 



