MORPHOLOGICAL REVISION OF THE SUBORDER. 



lOI 



Fig. 



and well-developed semicircular canals. A displaced portion of the petrosal shows 

 the penetration of the canals into its body. 



T\\&jiigal forms the lower half of the orbital rim. The orbital edge is widened 

 by the development of a strong, sharp ridge on the outer side of the bone so that the 

 socket is bordered on the lower side by a shelf of at least a centimeter in width. 

 The lower part of the bone is very thin and the edges are without thickening rugosi- 

 ties. On the inner side of the jugal a strong ridge extends obliquely downwards and 

 forwards from the orbit to the antero-inferior angle of the bone ; here it leaves the bone 

 and extends as a sessile process with a bifurcate end ; into the bifurcation of the end 

 articulates the upper end of the transverse, fig. 29. The articulation with the maxillary 

 is by a close interdigitating suture which locks the bones very closely together. 



The bones of the top of the skull have 

 already been described from specimens Nos. 

 I and 1002 and the separate elements figured, 

 but in the specimen No. looi the top of the 

 skull is preserved on one side without dis- 

 tortion and the bones can be seen in their 

 natural relations, plate 10, figs, i and 2. 



The postorbital consists of a flat ante- 

 rior portion and two posterior branches. One 

 of the posterior branches extends downwards 

 to join the jugal and form the upper half of 

 the posterior rim of the orbit ; it passes in- 

 side of the jugal and so forms much more of 

 the orbital rim than appears on the exterior. 

 The second, upper, posterior process passes 

 backward to join the prosquamosal and form the upper edge of the inferior temporal 

 vacuity. The anterior portion joins the postfrontal and parietal; its outer edge is 

 thickened and rugose and forms the posterior portion of the superorbital ridge. 



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, on the lower surfaces of the frontal and postfrontal 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 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 impos- 

 sible to trace the suture out upon the facial portion of the skull. Howse and Swin- 

 nerton 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 

 Pelycosauria; certainly, if it did, it very early coalesced with the surrounding bones. 

 '^\i^ frontal is an elongate bone lying horizontally in the skull ; near the pos- 

 terior 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 arrangement 

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

 plate 10, fig. I. 



29, — Inner view of suborbital portion of skull of 

 £). incisicus. No. 1001 University of Chicago. 

 Showing relations of the transverse, pterygoid, 

 maxillary, and palatine. X H- 



