A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 37 



quite thin, but there are no vacuities. The postfrontals bound the orbit above and are 

 placed between the frontal, parietal and postorbital. They are approximately quad- 

 rangular in outline. Posteriorly they are joined to the postorbital by a broad flaring 

 suture, the ridge thus formed marking the upper posterior angle of the orbital rim. The 

 external edge, which forms part of the orbit, is thickened and rounded. The postorbitals 

 (PL I, Fig. 7) take part in the formation of the orbit behind. They are united with the 

 postfrontals and parietals above, send a posterior process to the pro squamosal. The post- 

 orbital is crescentic in outline, thin antero-posteriorly and expanded laterally. The jugal 

 overlies a large part of the posterior surface of the lower portion and r eceives the lower 

 end into a notch on its inner surface, so that the postorbital takes little part in the rim of 

 the orbit, but forms a large part of its posterior wall. A deep groove extends on the 

 external side between the faces for the postfrontal and jugal and notches the rim. The 

 groove ends internally against a ridge which supports a long posterior process bearing the 

 face for the prosquamosal. Internal to this ridge there is a flat slightly sigmoid face for 

 the parietal. 



The maxillaries are very large bones, very thin at the upper and lateral parts. 

 They extend as thin j)lates far up on the side of the skull, reaching the lachrymals, pre- 

 frontals and nasals. They form a large part of the inferior and posterior border of the 

 nasal openings. Below they become suddenly very thick to contain the deep alveoli of 

 the teeth. This thickened portion takes the form of a strong ridge on the inner face of 

 the lower edge and extends from the anterior end nearly to the posterior. The lower 

 edge is gently convex downwards, anteriorly it is excavated by a deep notch, which marks 

 the point of union of the maxillary and premaxillary. There are seventeen teeth in the 

 maxillary. Two small ones occupy the maxillary part of the notch ; succeeding this are 

 two large teeth, one just appearing, and finally teeth nearly equal in size, but much 

 smaller than the canine. The canine is directed nearly straight downwards, more convex 

 on the outside than the inner and has well-defined anterior and posterior edges. The 

 succeeding teeth are nearly uniform in size except the last few, which diminish rapidly. 

 They are stout, quite strongly recurved and have- strong antero-posterior cutting edges 

 which are finely serrated. The maxillaries are joined with the premaxillary in front, 

 the nasal and prefrontal above, the lachrymal and jugal behind ; at the inner rounded and 

 thickened lower part is found a very distinct suture for the palatines. 



Measurements of Maxillary. 



M. 



Length ( nearly complete) 0.192 



Length of largest tooth from hase 0.055 



Length of largest tooth from outer edge of jaw 0.040 



Breadth of the same tooth at base 0.019 



Length of fifth tooth from outer edge 0.018 



