THE SKELETON IN MAMMALIA. VERTEBRAL COLUMN. 453 



but as the tail is followed back, the centra gradually 

 lengthen and become cylindrical, and at the same time the 

 neural arches and all the processes gradually become reduced 

 and disappear, so that the last few vertebrae consist of simple 

 rod-like centra. Chevron bones are frequently well-developed. 



Of the MONOTREMES Echidna has twelve caudal vertebrae, 

 two of which bear irregular chevron bones. In Ornitho- 

 rhynchus there are twenty or twenty-one caudal vertebrae with 

 well-developed hypapophyses, but no chevron bones. 



In MARSUPIALS there is great diversity as regards the tail. 

 In the Wombat and Koala the tail is small and without 

 chevron bones. In most other Marsupials it is very long, 

 having sometimes as many as thirty-five vertebrae in the 

 prehensile-tailed opossums. In the Kangaroos the tail is very 

 large and stout. Chevron bones are almost always present, and 

 in Notoryctes are large and expanded. 



Most EDENTATES have large tails with well -developed 

 chevron bones. The length of the tail varies greatly from the 

 rudimentary condition in Sloths to that in the Pangolins, one 

 of which has forty-six to forty-nine caudal vertebrae the 

 largest number in any known mammal. Chevron bones are 

 much developed, sometimes they are Y-shaped, sometimes as in 

 Priodon, they have strong diverging processes. The caudal 

 vertebrae of Glyptodonts, though enclosed in a continuous 

 bony sheath, have not become ankylosed together. 



The SIRENIA have numerous caudal vertebrae with wide 

 transverse processes. In the CETACEA also the tail is much 

 developed, and the anterior vertebrae have large chevron 

 bones and prominent straight transverse processes ; the pos- 

 terior caudal vertebrae, which in life are enclosed in the hori- 

 zontally expanded tail fin, are without transverse processes. 



In UNGULATA the tail is simple, formed of short cylindrical 

 vertebrae, which in living forms are never provided with 

 chevron bones. The number of caudal vertebrae varies from 

 four, sometimes met with in Procavia, to thirty-one in the 



