The first two vertebrae of the series are always very different from those which follow. The first is termed the {22} atlas, and articulates with the skull. The most remarkable fact about this bone (shared, however, by lower Vertebrates) is that its centrum is detached from it and attached to the next vertebra, in connexion with which it will be referred to immediately. The whole bone thus gets a ring-like form, and the salient processes of other vertebrae are but little developed, with the exception of the transverse processes, which are wide and wing-like. In many Marsupials, such as the Wombat and Kangaroo, the arch of the atlas is open below, there being no centre of ossification. In others, such as _Thylacinus_, there is a distinct nodule of bone in this situation not concrescent with the rest of the arch. [Illustration] [Illustration] FIG. 7.--Human atlas (young), showing development. x 3/4. _as_, Articular surface for occiput; _g_, groove for first spinal nerve and vertebral artery; _i a_, inferior arch; _t_, transverse process. (From Flower's _Osteology_.) FIG. 8.--Inferior surface of atlas of Dog. x 1/2. _sn_, Foramen for first spinal nerve; _v_, vertebrarterial canal. (From Flower's _Osteology_.) [Illustration] FIG. 9.--Atlas of Kangaroo.... (From Parker and Haswell's _Zoology_.) The second vertebra, which is known as the axis or epistropheus, is a compound structure, the anterior "odontoid process," which fits into the ring of the atlas, being in reality the detached centrum of that vertebra.[11] It is a curious fact about that process that it has independently become spoon-shaped in two divisions of Ungulates; that it has become so seems to be shown by the fact that in the earlier types of both it has the simple peg-like form, which is the prevailing form. The cervical {23} vertebrae are occasionally wholly (Right Whales) or partially (many Whales, Jerboa, certain Edentates) welded into a combined mass. Indications of this have even been recorded in the human subject. [Illustration] [Illustration] FIG. 10.--Side view of axis of Dog. x 2/3. _o_, Odontoid process; _pz_, posterior zygapophysis; _s_, spinous process; _t_, transverse process; _v_, vertebrarterial canal. (From Flower's _Osteology_.) FIG. 11.--Anterior surface of axis of Red Deer. x 2/3. _o_, Odontoid process; _pz_, posterior zygapophysis; _sn_, foramen for second spinal nerve. (From Flower's _Osteology_.) The dorsal vertebrae vary greatly in number: nine (_Hyperoodon_) seems to be the lowest number existing normally; while there may be as many as nineteen, as in _Centetes_, or twenty-two, as in _Hyrax_. These vertebrae are to be defined by the fact that they carry ribs, and the first one or two lumbars are often "converted into" dorsals by the appearance of a small supernumerary rib. The spinous processes of these vertebrae are commonly long, and sometimes very long. It is only among the Glyptodons that any of these vertebrae are fused together into a mass.