The basisphenoid (fig. 45, 12) is an unpaired wedge-shaped bone, united along a deep vertical suture with the basi-occipital. The two bones are, however, partially separated in the mid-ventral line by a foramen, the opening of the median Eustachian canal, which leads into a complicated system of Eustachian passages ultimately communicating with the tympanic cavity.

The dorsal surface of the basisphenoid is well seen in a section of the skull, but owing to the way it tapers ventrally, it appears on the ventral surface only as a very narrow strip of bone wedged in between the basi-occipital and pterygoids. In a lateral view it is seen to be drawn out in front into an abruptly truncated process, the rostrum, which forms part of the interorbital septum. On the anterior part of the dorsal surface is a deep pit, the pituitary fossa or sella turcica, at the base of which are a pair of foramina, through which the carotid arteries pass. Dorsolaterally the basisphenoid articulates with the alisphenoids.

Fig. 44. Lateral view of the skull of an Alligator (Caiman latirostris). × 1/3. (Brit. Mus.)

1. premaxillae.9. palatine.16. external auditory22. angular.
2. maxillae.10. pterygoid.meatus.III, VI, opening for exit
3. lachrymal.11. transpalatine.17. frontal.of oculomotor and
4. prefrontal.12. quadratojugal.18. supra-angular.abducens nerves.
5. jugal.13. exoccipital.19. articular.V, foramen ovale.
6. postfrontal.14. basi-occipital.20. dentary.X, pneumogastric foramen.
7. squamosal.15. foramen by which carotidXII, hypoglossal foramen.
8. quadrate.artery enters skull.21. coronoid.

The alisphenoids (fig. 45, 13) are a pair of irregular bones which arise from the basisphenoid antero-laterally, and are united dorsally with the parietal, frontal, and postfrontals. They bound most of the anterior part of the brain case, and each presents on its inner face a deep concavity which lodges the cerebral hemisphere of its side. Viewed from the ventral side the two alisphenoids are seen to almost or quite meet one another immediately below the frontal, and then to diverge, forming an irregular opening—partially closed by cartilage in the fresh specimen,—through which the optic nerves leave the cranial cavity. Further back the alisphenoids meet one another for a narrow area, and then diverge again, so that between each and the rostrum of the basisphenoid there appears an opening (fig. 44, III, VI) through which the oculomotor and abducens nerves leave the cranium. Further back still each is united for a short space with the basisphenoid, pterygoid and quadrate, and then becomes separated from the quadrate by a large foramen, the foramen ovale (fig. 44, V), through which the whole of the trigeminal nerve passes out.

The dorsal portion of the parietal segment is formed by the parietal (fig. 45, 4), which though double in the embryo, early comes to form a single bone. It extends over the posterior part of the cranial cavity, and is continuous in front with the frontal, behind with the supra-occipital, and laterally with the postfrontals, squamosals, alisphenoids, pro-otics and epi-otics. It forms the inner boundary of a large rounded vacuity on the roof of the skull, the supratemporal fossa.

The frontal segment is very imperfectly ossified, there being no certain representatives of either the ventral member, the presphenoid, or the lateral members, the orbitosphenoids. On the dorsal side there is, however, a large development of membrane bones. There is a large frontal (fig. 45, 3), unpaired, except in the embryo, united behind with the parietal and postfrontal, and drawn out in front into a long process which is overlapped by the prefrontals and posterior part of the nasals. The frontal ends off freely below, owing to the orbitosphenoidal region being unossified, it forms a considerable part of the roof of the cranial cavity, but takes no part in the formation of the wall.

Each prefrontal (fig. 45, 14) forms part of the inner wall of the orbit and sends ventralwards a process which meets the palatine.