We may pass now to the special characters of the cervical vertebrae in the different groups. In MONOTREMES and MARSUPIALS the number of cervical vertebrae is always seven. With the exception of the atlas of _Echidna_ the cervical vertebrae of Monotremes are without zygapophyses. In Monotremes the transverse processes ossify from centres distinct from that forming the body, and remain suturally connected with the rest of the vertebra until the adult condition is reached. The method of the ossification of the atlas in Marsupials varies considerably, thus in some forms such as the Wombats (_Phascolomys_) there is an unossified gap in the middle of the inferior arch of the atlas, which may remain permanently open; in _Thylacinus_ this gap is filled up by a distinct heart-shaped piece of bone, while in _Didelphys_ and _Perameles_ the atlas is ossified below in the same way as in other mammals. In _Notoryctes_ the second to sixth cervical vertebrae are ankylosed together. The cervical vertebrae of the EDENTATA have some remarkable peculiarities. In the three-fingered Sloth, _Bradypus_, there are nine cervical vertebrae, all except the last of which have their transverse processes perforated by the vertebrarterial canals. In a two-fingered sloth, _Choloepus hoffmanni_, there are only six cervical vertebrae. In the Megatheriidae, Anteaters (Myrmecophagidae), Pangolins (Manidae), and Aard Varks (Orycteropodidae), the cervical vertebrae are normal, but in the Armadillos (Dasypodidae), and still more in the Glyptodonts, several of them are commonly fused together. The fusion affects not only the centra, but also the neural arches, so that the neural canals form a continuous tube. In the Glyptodonts there is a complex joint at the base of the neck to allow the partial retraction of the head within the carapace. This arrangement recalls that in Tortoises. As a rule the SIRENIA possess seven short cervical vertebrae, not fused together and not presenting any marked peculiarities. In _Manatus_ however there are only six cervical vertebrae and they are very variable. [Illustration FIG. 87. CERVICAL VERTEBRAE OF A YOUNG FIN WHALE (_Balaenoptera musculus_) × 1/10. (Camb. Mus.) 1. surface on the atlas for articulation with the occipital condyle of the skull. 2. foramen for exit of the first spinal nerve. 3. upper transverse process. 4. lower transverse process. In the fresh specimen these two transverse processes are united by cartilage, in adult individuals the whole transverse process is ossified. 5. epiphyses of centrum. 6. neural spine.]