b. The dermal exoskeleton is strongly developed, and is combined with endoskeletal structures derived from the ribs and vertebrae to form the carapace.
The Carapace (fig. 36) consists of a number of plates firmly united to one another by sutures. They have a very definite arrangement and include:
(a) the nuchal plate (fig. 36, 1), a wide plate forming the whole of the anterior margin of the carapace. It is succeeded by three series of plates, eight in each series, which together make up the main part of the carapace. Of these the small
(b) neural plates[1] (fig. 36, A, 2) form the middle series. They are closely united with the neural arches of the underlying vertebrae;
(c) the costal plates[89] (fig. 36, A, 3) are broad arched plates united to one another by long straight sutures. They are united at their inner extremities with the neural plates, but the boundaries of the two sets of plates do not regularly correspond. Each is united ventrally with a rib which projects beyond it laterally for some distance; (d) the marginal plates (fig. 36, 4) are twenty-three in number, eleven lying on each side, while an unpaired one lies in the middle line posteriorly. Many of them are marked by slight depressions into which the ends of the ribs fit; (e) the pygal plates (fig. 36, 5) are two unpaired plates lying immediately posterior to the last neural.
Fig. 36. A, dorsal and B, ventral view of the carapace of a Loggerhead Turtle (Thalassochelys caretta), (after Owen).
| 1. nuchal plate. | 6. rib. |
| 2. first neural plate. | 7. thoracic vertebra. |
| 3. second costal plate. | 8. first vertebral shield. |
| 4. marginal plate. | 9. costal shield. |
| 5. pygal plate. |
The sculpturing due to the epidermal shields is very obvious on the carapace.
The plastron (fig. 37) consists of one unpaired ossification, the entoplastron, and four pairs of ossifications called respectively the epiplastra, hyoplastra, hypoplastra, and xiphiplastra.