1. epiplastron (clavicle). 2. entoplastron (interclavicle). 3. hyoplastron. 4. hypoplastron. 5. xiphiplastron.] II. ENDOSKELETON. 1. THE AXIAL SKELETON. The axial skeleton includes the vertebral column, the ribs, and the skull. A. THE VERTEBRAL COLUMN AND RIBS. The number of vertebrae in the Green Turtle is thirty-eight, not a great number as compared with that in many reptiles, and of these eighteen are caudal. The vertebral column is divisible into four regions only--=cervical=, =thoracic=, =sacral=, and =caudal=. THE CERVICAL VERTEBRAE. These are eight in number, and are chiefly remarkable for the great variety of articulating surfaces which their centra present, and for their mobility upon one another. The first or =atlas= vertebra differs much from all the others and consists of the following parts:-- _a._ the =neural arch=, formed of two separate ossifications united in the mid-dorsal line; _b._ the =inferior arch=; _c._ the =centrum=, which is detached from the rest and forms the odontoid process of the second vertebra. Each half of the =neural arch= consists of a ventral portion, the =pedicel=, which lies more or less vertically and is united ventrally to the inferior arch, and of a dorsal portion, the =lamina=, which lies more or less horizontally and meets its fellow in the middle line in front, partially roofing over the neural canal. Each pedicel bears a facet on its anterior surface, which, with a corresponding one on the inferior arch, articulates with the occipital condyle of the skull. Three similar facets occur also on the posterior surface of the pedicel and inferior arch, and articulate with the odontoid process. The laminae meet one another in front, but do not fuse, while behind they are separated by a wide triangular space. They bear a pair of small downwardly-directed facets, the =postzygapophyses=, for articulation with the prezygapophyses of the second vertebra. The =inferior arch= is a short irregular bone bearing two converging facets for articulation with the occipital condyle and odontoid process respectively. The =centrum= or =odontoid process= has a convex anterior surface for articulation with the neural and inferior arches, and a concave posterior surface by which it is united with the centrum of the second or axis vertebra. It bears posteriorly a small epiphysis which is really a detached portion of the inferior arch.