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.
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.
[Illustration FIG. 39. LONGITUDINAL VERTICAL SECTION THROUGH THE CRANIUM OF A GREEN TURTLE (_Chelone midas_). × 2/3. (Camb. Mus.)
1. parietal. 2. squamosal. 3. quadrate. 4. basisphenoid. 5. basi-occipital. 6. quadratojugal. 7. pro-otic. 8. opisthotic. 9. pterygoid. 10. palatine. 11. rod passed into narial passage. 12. exoccipital. 13. epi-otic fused to supra-occipital. 14. supra-occipital. 15. premaxillae. 16. maxillae. 17. jugal. 18. postfrontal. 19. vomer. 20. prefrontal. 21. frontal. V, VII, VIII, IX, X, XI, XII, foramina for the exit of cranial nerves.]
Skeletal structures occur in connection with each of the three special sense organs of hearing, sight, and smell.
The auditory or periotic capsule of the turtle is rather large and its walls are well ossified, epi-otic, pro-otic and opisthotic bones being present.
The =epi-otic= (fig. 39, 13) is the more dorsal of the three bones, and in the adult is completely ankylosed with the supra-occipital.