The ventral portion of the parietal segment is formed by the basisphenoid (figs. 38 and 39, 4) which lies immediately in front of the basi-occipital. A triangular portion of it is seen in a ventral view of the skull, but it is quickly overlapped by the pterygoids. It gives off dorsally a pair of short processes which meet the pro-otics.
The alisphenoidal region is unossified and the only other constituents of the parietal segment are the parietals (fig. 39, 1). These are large bones which, after roofing over the cranial cavity, extend upwards and become expanded into a pair of broad plates which unite with the squamosal and bones of the frontal segment to form a wide, solid, false roof to the skull. Each also sends ventralwards a plate which meets an upgrowth from the pterygoid and acts as an alisphenoid.
The Frontal segment.
Of the frontal segment the basal or presphenoidal and lateral or orbitosphenoidal portions do not become ossified, the dorsal portion however includes three pairs of membrane bones, the frontals, prefrontals and postfrontals.
The frontals are a pair of small bones lying immediately in front of the parietals, and in front of them are the prefrontals (figs. 38 and 39, 20), a pair of similar but still smaller bones, which are produced ventrally to meet the vomer and palatines. They form also the dorsal boundary of the anterior nares. The postfrontals (figs. 38 and 39, 18) are larger bones, united dorsally to the frontals and parietals, posteriorly to the squamosals, and ventrally to the jugals and quadratojugals. All three pairs of frontal bones, especially the postfrontals, take part in the bounding of the orbits.
Fig. 39. Longitudinal vertical section through the Cranium of a Green Turtle (Chelone midas). × 2/3. (Camb. Mus.)
| 1. parietal. | 10. palatine. | 17. jugal. |
| 2. squamosal. | 11. rod passed into narial | 18. postfrontal. |
| 3. quadrate. | passage. | 19. vomer. |
| 4. basisphenoid. | 12. exoccipital. | 20. prefrontal. |
| 5. basi-occipital. | 13. epi-otic fused to | 21. frontal. |
| 6. quadratojugal. | supra-occipital. | V, VII, VIII, IX, X, XI, XII, |
| 7. pro-otic. | 14. supra-occipital. | foramina for the exit of |
| 8. opisthotic. | 15. premaxillae. | cranial nerves. |
| 9. pterygoid. | 16. maxillae. |
2. The Sense capsules.