At the posterior end of the trunk in all mammals a certain number of vertebrae are found fused together forming the sacrum. But of these only two or three answer to the definition of true sacral vertebrae in being united to the ilia by small ribs. The others which belong to the caudal series may be called pseudosacral vertebrae. In different individuals of the same species it sometimes happens that different vertebrae are attached to the pelvis and form the sacrum. Sometimes even different vertebrae are attached to the pelvis at successive periods in the life history of the individual. This is owing to a shifting of the pelvis and has been especially well seen in man. In young human embryos the pelvis is at a certain stage attached to vertebra 30, but as development goes on it becomes progressively attached to the twenty-ninth, twenty-eighth, twenty-seventh, twenty-sixth and twenty-fifth vertebrae. As the attachment to these anterior vertebrae is gained, the attachment to the posterior ones becomes lost, so that in the adult the pelvis is generally attached to vertebrae 25 and 26. But there are no absolutely pre-determined sacral vertebrae, as sometimes the pelvis does not reach vertebra 25, remaining attached to vertebrae 26 and 27; sometimes it becomes attached even to vertebra 24. This shifting of the pelvis is seen in _Choloepus_ in a more marked degree even than in man. 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.