Of the Monotremata, Ornithorhynchus has two sacral vertebrae ankylosed together, while Echidna has three or four[164].
In Marsupialia as a rule only one vertebra is directly united to the ilia, but one or two more are commonly fused to the first. In the Wombats there may be as many as four or five vertebrae fused together in the sacral region. In Notoryctes there is extensive fusion in the sacral region, six vertebrae, owing mainly to the great development of their metapophyses, being united with one another, and with the ilia, and the greater part of the ischia.
In most Edentata there is an extensive fusion of vertebrae in the sacral region. This is especially marked in the Armadillos and Megatheriidae, and to a less extent in the Sloths and Aard Varks.
In the Sirenia the vestigial pelvis is attached by ligament to the transverse processes of a single vertebra, which hence may be regarded as sacral.
In Cetacea there is no sacrum, the vestigial pelvis not being connected with the vertebral column.
In most Ungulata the sacrum consists of one large vertebra united to the ilia, and having a varying number of smaller vertebrae fused with it behind.
The same arrangement obtains in most Rodentia, but in the Beavers (Castoridae) all the fused vertebrae are of much the same size, the posterior ones having long transverse processes which nearly meet the ilia.
In Carnivora there may be two sacral vertebrae as in the Hyaena, three as in the Dog, four or five as in Bears and Seals.
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.