(_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. [Illustration Fig. 36. A, DORSAL AND B, VENTRAL VIEW OF THE CARAPACE OF A LOGGERHEAD TURTLE (_Thalassochelys caretta_), (after Owen). 1. nuchal plate. 2. first neural plate. 3. second costal plate. 4. marginal plate. 5. pygal plate. 6. rib. 7. thoracic vertebra. 8. first vertebral shield. 9. costal shield.] 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=. 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. [Illustration FIG. 37. THE PLASTRON OF A GREEN TURTLE (_Chelone midas_). × 1/7. (Camb. Mus.)