The Vertebrate Skeleton

by Sidney H. Reynolds

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.

Most of the Carnivora except the Bears and Seals have very long tails, the greatest number of vertebrae, thirty-six, being met with in Paradoxurus. Bears have only eight to ten caudal vertebrae. Chevron bones are not often much developed.

In Insectivora the tail is very variable as regards length, the number of vertebrae varying from eight in Centetes to forty-three in Microgale.

In Chiroptera the tail is sometimes quite rudimentary, and as in Pteropus, composed of a few coalesced vertebrae, sometimes it is formed of a large number of slender vertebrae.

In Primates also the tail is very variable. In Man all the four caudal vertebrae are rudimentary and are fused together, forming the coccyx. In the Anthropoid apes, too, there are only four or five caudal vertebrae. In many monkeys of both the eastern and western hemispheres the tail is very long, having thirty-three vertebrae in Ateles, in which genus it is also prehensile. Chevron bones are present in all Primates with well-developed tails. In the Lemuroidea the number of caudal vertebrae varies from seven to twenty-nine.