In INSECTIVORA from three to five are united, while in many CHIROPTERA all the sacral and caudal vertebrae have coalesced. Among PRIMATES, in Man and Anthropoid Apes there are usually five fused vertebrae forming the sacrum, but of these only two or three are connected to the ilia by ribs. In most of the other Anthropoidea there are two or three fused vertebrae, and in the Lemuroidea two to five. FREE CAUDAL VERTEBRAE. The free caudal vertebrae vary greatly in number and character. When the tail is well developed, the anterior vertebrae are comparatively short and broad, with well-developed neural arches and zygapophyses; 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 _Ornithorhynchus_ 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 posterior caudal vertebrae, which in life are enclosed in the horizontally 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 Elephant. The tail is exceedingly long in _Anoplotherium_ and in _Phenacodus_, in which there are thirty caudal vertebrae. In RODENTIA the tail is variable. In the Hares, Guinea pig (_Cavia_) and _Capybara_ it is very small, in _Pedetes_ and the Beaver it is very long and has well-developed chevron bones.