II. Dermal exoskeletal structures.
| 1. | Dentine and cement of teeth. |
| 2. | Bony scutes of Armadillos. |
ENDOSKELETON.
Vertebral Column.
Cervical Vertebrae.
The cervical vertebrae of all mammals have certain characters in common. However long the neck may be, the number of cervical vertebrae, with very few exceptions, is seven. Movable ribs are generally absent, and if present are small and do not reach the sternum. The transverse processes are generally wide but not long, and are perforated near the base by the vertebrarterial canals, through which the vertebral arteries pass; they generally bear downwardly-directed inferior lamellae which are sometimes as in the seventh human cervical seen to ossify from centres distinct from those forming the rest of the transverse process, and are really of the nature of ribs. The atlas and axis always differ much from the other vertebrae.
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.