[2] The border of the squamous part is thick, strongly serrated, bevelled at the expense of the inner table above, where it rests upon the parietal bones, and at the expense of the outer table on either side, where it receives the lateral pressure of those bones; this border is continued below into a triangular, rough surface, which articulates with the great wing of the sphenoid.

The posterior borders of the orbital plates are thin and serrated, and articulate with the small wings of the sphenoid.

[3] The frontal bone is ossified in membrane from two primary centers, one for each half, which appear toward the end of the second month of fetal life, one above each supraorbital margin.

At birth the bone consists of two pieces, separated by the frontal suture, which is usually obliterated by Intramembranous ossification, except at its lower part, by the eighth year, but occasionally persists throughout life.

The sinuses are of considerable size by the seventh or eighth year, but do not attain their full proportions until after puberty.

Instead, in reptiles, bony fish and amphibians it is often separated from the orbits by one or two additional bones not found in mammals.