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S. W. WILLISTON 



a typically lower opening bounded above by the postorbito- 

 squamosal arcade. In none of these forms does the quadra to jugal 

 enter into the opening; in some it has entirely disappeared; and 

 the former, I believe, was the primitive condition. The opening, 

 I believe, first appeared below and behind the orbit, at the apex of 

 the squamosal. In the Cotylosauria the squamosal is a large bone 

 extending far down on the side of the skull and back of the quad- 

 rate. In Dimetrodon and Sphenacodon, indeed, the quadratojugal 

 is almost confined to the posterior side of the quadrate. Its 

 tendency was to disappear in this type of skull, and only in some 

 Diapsida did it become a part of the lower arch. 



Fig. 3. — Mycterosaurus, skull, natural size. Synapsida, Permocarboniferous 



It seems now evident that the temporal openings arose in yet 

 another way: by the primitive separation of the postorbito- 

 squamosal arcade from the parietal in the stegocrotaphous skull; 

 and it seems very probable that this type of skull arose very early 

 in geological history, as early as the lower opening, and before the 

 separation of the upper arch in the diapsid skull. In these forms, 

 or in most of them at least, an additional temporal bone was 

 retained long after it was lost in other groups. And this is one 

 of the reasons why I believe that the Ichthyosauria and the Squa- 

 mata arose from a common or allied stem, direct from the Coty- 

 losauria. For this phylum I propose the name Parapsida. As 

 we have seen, Baur and others, because of the many primitive 

 characters of the Ichthyosauria, believed that the order came from 

 the original double-arched stem, that the lower temporal opening 

 had been secondarily closed. Cope, in 1896, asserted that the 



