In the floor of the skull, in the region of the pituitary body, there is formed a basisphenoid; while in the lateral parts of the wall of this part of the cranium, there is a bone known as the alisphenoid.
In front, parts of the lateral walls of the cranium ossify as the orbitosphenoids.
In view of the very imperfect ossification of the cartilaginous cranium of the Dipnoi, and of the fact that there is certainly no direct genetic connection between the Teleostei on the one hand, and the Amphibia and Amniota on the other, it is very difficult to believe that most of the ossifications of the cranium in the Amphibia and Amniota have more than a general correspondence with those in the Teleostei.
In the Amphibia the ossifications in the cartilage are comparatively few. In the occipital region there is a lateral ossification on each side of the exoccipital, the basioccipital region being unossified, and the supraoccipital at the utmost indurated by a calcareous deposit.
The periotic capsule is ossified by a prootic centre, which meets the exoccipital behind.
The front part of the cartilaginous cranium is ossified by a complete ring of bone—the sphenethmoid bone—which embraces part of the ethmoid region, and of the orbitosphenoid and presphenoid regions.
In the Amphibia the cartilaginous cranium, with its centres of ossification, is easily separable from the membranous investing bones.
In the Amniota the cartilaginous cranium, whose development in the embryo has already been described, becomes in the adult much more largely ossified, and the bones which replace the primitive cartilage unite with the membrane bones to form a continuous bony cranium.
The centres of ossification become again much more numerous. In the occipital segment analogous centres to those of Teleostei are again found; and it is probable that the exoccipitals are homologous throughout the series, the supraoccipital and basioccipital bones of the higher types being merely identical in position with the similarly named bones in Fishes.
In the periotic there are usually three centres of ossification, first recognised by Huxley. These are the prootic, the epiotic and opisthotic, the situations of which have already been defined. Of these the prootic is the most constant.