OF THE PELYCOSAURIAN GENUS DIMETRODON. 15 



large campullar space {ampuUenratim Siebenrock) and well developed semicircular 

 canals. A displaced portion of the petrosal shows the penetration of the canals into 

 its body. 



The jugal: The jugal 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 

 rugosities. 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 



Fig. 3. View of the inner side of the skull opposite the posterior end of the maxillary showing the mode of articu- 

 lation of jugal, palatine, maxillary and transverse ; pt. transverse. Specimen No. 1002. 



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

 end articulates the upper end of the transverse, figure 3. 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 

 number 1 and 114 and the separate elements figured but in the specimen 1001 the 

 top of the skull is preserved on one side without distortion and the bones can be seen 

 in their natural relations. Figures 1 and la, PI. VI. 



The postorbital : The postorbital consists of a flat anterior j^ortion and two post- 

 erior 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 inside 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 infei'ior 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. 



