The epiplastra (fig. 37, 1) are the most anterior, they are expanded and united to one another in the middle line in front, while behind each tapers to a point which lies external to a process projecting forwards from the hyoplastron. They are homologous with the clavicles of other vertebrates.
The entoplastron or episternum (fig. 37, 2) which is homologous with the interclavicle of other reptiles, is expanded at its anterior end and attached to the symphysis of the epiplastra, while behind it tapers to a point and ends freely.
The hyoplastra are large irregular bones each closely united posteriorly with the corresponding hypoplastron, and drawn out anteriorly into a process which lies internal to that projecting backwards from the epiplastron. Each gives off on its inner surface a slender process which nearly meets its fellow, while the anterior half of the outer surface is drawn out into several diverging processes.
The hypoplastra (fig. 37, 4) are flattened bones resembling the hyoplastra, with which they are united by long sutures; the posterior half of both outer and inner surfaces is drawn out into a number of pointed processes.
The xiphiplastra are small flattened elongated bones meeting one another in the middle line posteriorly. In front they are notched and each interlocks with a process from the hypoplastron of its side. The hyoplastra, hypoplastra and xiphiplastra are homologous with the abdominal ribs of Crocodiles.
Fig. 37. The Plastron of a Green Turtle (Chelone midas). × 1/7. (Camb. Mus.)
| 1. epiplastron (clavicle). | 4. hypoplastron. |
| 2. entoplastron (interclavicle). | 5. xiphiplastron. |
| 3. hyoplastron. |
II. ENDOSKELETON.
1. The Axial Skeleton.