To each thoracic vertebra from the second to ninth inclusive, there corresponds a pair of =ribs= (fig. 36, 6) of a rather special character. Each is suturally united with the anterior half of the edge of its own vertebra, and overlaps on to the posterior half of the edge of the next preceding vertebra. The ribs are much flattened, and each is fused with the corresponding costal plate, beyond which it projects to fit into a pit in one of the marginal plates. The tenth thoracic vertebra is smaller than the others, and its neural arch does not overlap the preceding vertebra, it bears a pair of small ribs which are without costal plates, but meet those of the ninth vertebra. There are no =lumbar= vertebrae. THE SACRAL VERTEBRAE. The =sacral vertebrae= are two in number, they are short and wide, their centra are ankylosed together, and their neural arches are not united to the carapace. The first has the anterior face of the centrum concave and the posterior flat, while both faces of the second are flat. Each bears a pair of short ribs which meet the ilia, but are not completely ankylosed either with them or the centra. THE CAUDAL VERTEBRAE. The =caudal vertebrae= are eighteen in number. The centrum of the first is flat in front and is ankylosed to the second sacral; behind it is convex. The others are all very similar to one another, and decrease gradually in size when followed back. Each has a moderately long centrum, concave in front and convex behind, both terminations being formed by epiphyses. The neural arch arises only from the anterior half of the vertebra; it bears a blunt truncated neural spine and prominent pre- and post-zygapophyses. The first seven caudal vertebrae bear short ribs attached to their lateral margins, the similar outgrowths on the succeeding vertebrae do not ossify from distinct centres, and are transverse processes rather than ribs. B. THE SKULL. The skull of the Turtle is divisible into the following three parts:-- (1) the cranium; (2) the lower jaw or mandible; (3) the hyoid. (1) THE CRANIUM. The =cranium= is a very compact bony box, containing a cavity in which the brain lies, and which is a direct continuation of the neural canal of the vertebrae. [Illustration FIG. 38. THE SKULL OF THE GREEN TURTLE (_Chelone midas_). × 1/2. A, POSTERIOR HALF, B, ANTERIOR HALF. (Brit. Mus.) 1. parietal. 2. squamosal. 3. quadrate. 4. basisphenoid. 5. basi-occipital. 6. quadratojugal. 7. opisthotic. 8. exoccipital. 9. foramen magnum. 10. splenial. 11. articular. 12. dentary. 13. angular. 14. supra-angular. 15. premaxilla. 16. maxilla. 17. jugal. 18. postfrontal. 19. vomer. 20. prefrontal. 21. frontal. 22. external auditory meatus leading into tympanic cavity.] Like those of the skull as a whole its component bones may be subdivided into three sets:-- 1. those forming the brain-case or =cranium proper=;