480 ALLIS. [Vo. II. 
the bones immediately behind it are slightly bevelled, thus form- 
ing a shelving surface which looks upward and outward, and, 
when the cheeks are distended, serves as an articular surface for 
the overlapping edge of the upper postorbital. This lateral rib 
or thickening only extends one-half the length of the bone, 
which behind it is much smaller and narrower, the lateral edge 
and under surface being cut away by the postorbital process, a 
small portion of which here comes to the level of the outer 
surface of the dermal bones (Cut 2). 
The postorbital ossification has roughly the shape of an 
inverted pyramid, one edge of which projects laterally and forms 
the hind boundary of the orbit. The upper outer angle of the 
ossification expands into an elongate cap-like piece, which over- 
hangs the pyramid in front, behind, and laterally. Its outer 
surface is divided into two portions by a strong longitudinal 
line, which is continuous in front with the lower outer edge 
of the postfrontal, and behind with the corresponding edge of 
the squamosal. The part above this line is the small portion 
that forms a part of the external surface of the dermal bones, 
and it has in every particular the characteristic appearance of 
these bones. The part below the line lies at a deeper level and 
gives attachment to portions of the levator arcus palatini and 
adductor mandibule muscles. The line separating these two 
surfaces is sharp and strong, while that separating the upper 
surface from the dermal postfrontal is faint and indistinct, and 
might easily be overlooked. 
The postorbital ossification gives support on its upper surface 
to the frontal and squamosal as well as to the postfrontal, but 
neither of these bones are closely attached to it. The squamosal 
projects slightly under the frontal and postfrontal, and separates 
the main lateral canal from the spiracular canal, which opens 
widely on the upper surface of the skull immediately below it. 
The spiracular canal has been fully described by Sagemehl 
(No. 13, p. 200) and by Wright (No. 20, p. 492). 
The infra-orbital canal enters the postfrontal in front of the 
lateral edge of the postorbital ossification (Cut 2, zzf. c.), and 
no branch is given off in its course through it. At the median 
edge of the bone it forms an anastomosis with the supra-orbital 
canal, and a double system is given off; that is, one formed by 
the complete fusion of two systems, one belonging to each of 
