16 THE MORPHOLOGY OF THE SKULL 



The postfrontal : The postfrontal is a quadrangular bone which articulates 

 with postorbital and frontal, its outer edge carries forward the rugose superorbital 

 ridge. 



The roof of the orbit formed by the postorbital, postfrontal, frontal and prefrontal 

 is rounded and vaulted so that its capacity is much increased inwardly. From the 

 inner edges of the lower side of the postorbital and prefrontal, ridges extend inward 

 in a curve, these are continued inward on the lower surfaces of the frontal and post- 

 frontal until they finally meet on the median line of the skull completing a perfect 

 semicircle. This truss-like ridge surrounding the vaulted roof of the orbit adds greatly 

 to the strength of the skull. 



The lachrymal: The lachrymal is not well shown in any of the specimens nor is 

 there a lachrymal foramen. In some of the specimens there is evidence of a faint 

 suture on the anterior edge of the orbit indicating the possible presence of a distinct 

 bone but it is impossible to trace the suture out upon the facial portion of the skull. 

 Howse and Swinnerton in their discussion of the development of Sphenodon say that 

 there is no trace of a lachrymal in that form, it may be very possible that it did not 

 develop in the Pelycosaxma, certainly if it did it very early coalesced with the sur- 

 rounding bones. 



Th^ fronted : The frontal is an elongate bone lying horizontally in the skull, near 

 the posterior end a process extends outward to the orbital rim forming the middle of 

 the edge. The union of the bones of the two sides gives a distinct cruciform arrange- 

 ment in the middle of the skull roof The articulations of the bone are best shown in 

 Figure 1, PI. VI. 



The prefrontal : The prefrontal forms the superior anterior angle of the orbit and 

 extends forward between the nasal and frontal above and the maxillary and lachrymal 

 (?) below. The posterior portion of the bone is bent at right angles on the antero- 

 posterior axis, so that the upper portion of the bones is horizontal and the lower 

 vertical. The horizontal portion forms a part of the roof of the skull and the anterior 

 part of the superorbital ridge. On the vertical portion a strong ridge carries forward 

 onto the facial region the superorbital ridge. Beneath the posterior end of this ridge 

 and just anterior to the orbit is a deep pit. The presence of this ridge and pit is one 

 of the characteristic features of the Pelycosaurian skull. 



The nasal : The nasals are elongate bones occupying the median line of the skull 

 and extending from a point just anterior to the orbits to the anterior nares in front. 



The septo-maxillary : Anterior to the nasal and forming the posterior edge of the 

 narial opening is a singular bone, the septo-maxillary. These bones are of peculiar 

 form, difficult of description, but indicated in figures 1, Pis. II and IV. Each bone 



