28 THE ROYAL SOCIETY OF CANADA 



Postorbital. Is of about the size of the squamosal and is equally 

 heavy, with the same styles of sculpture covering its upper and border 

 surfaces. It is four-sided in outline, as seen from above, longer than 

 broad, narrower in front than behind, and lies external to the parietals 

 from the supratemporal fossa and the squamosal forward to the post- 

 frontal. It supplies about one-lifth of the margin of the supratemporal 

 fossa. 



Inferiorly it is longitudinally grooved, at a a short distance 

 internal to the exterior free border, for nearly two-fifths of its length 

 posteriorly • for the reception of the forwardly directed squamosal 

 process, leaving a smooth surfaced inframarginal area external to the 

 groove. This inframarginal area and the lower face of the squamosal 

 process together form a continuous arched surface over the latero- 

 temporal fossa with the assistance posteriorly of the squamosal to a 

 limited extent. Internal to the squamosal process the bone is ex- 

 cavated upward to form the outer half of the vaulted roof of the supra- 

 temporal cavity which leads posteriorly to the supratemporal fossa. 

 The postorbital, at its anterior end, enters slightly into the formation 

 of the orbital rim and the roof of the orbital cavity. Behind the 

 orbit, next to the external border, the bone thickens downward and, 

 judging from the shape of a broken surface here preserved, (plate 

 I, a) developed a descending process to constitute the upper part of a 

 postorbital bar separating the orbit from the latero temporal fossa. 

 Internal to the postorbital bar the orbital cavity is confluent with the 

 supratemporal cavity beneath an angulated transverse ridge running 

 inward from the postorbital to the side-wall of the brain-case (alisphen- 

 oid) and defining the limits of the two cavities. 



Postfrontal. This bone is small, thick, rhomboidal in outline as 

 seen from above, with the breadth about equal to the length. It lies 

 external to the frontal in advance of the postorbital, and has a rugose 

 sculpture similar to that of the latter element and of the squamosal. 

 Its external border enters the orbital rim. Its under surface is con- 

 cave, continuing outward from the frontal the vaulted roof of the 

 orbital cavity. In this surface are a number of small foramina like 

 those of the adjacent frontal surface but directed inward instead of 

 outward. At the front end of this bone are two suturai facets, one, 

 next to the frontal, short, and facing directly forward, the other twice 

 as long as the first, and facing obliquely outward and forward, between 

 it and the lateral free border (orbital rim). The first facet in 

 conjunction with the facet running forward and inward from it on the 

 frontal indicates the position, it is thought, of a prefrontal. The 



