1 88 S. W. WILLISTON. 



is the Pelycosaurian Diinctrodon (Fig. 5) from the Permian of 

 Texas, recently fully made known by Professor Case. 1 Here we 

 have, in addition to the fully developed superior opening, a large 

 fenestration below the squamoso-postorbital bar, and bounded 

 below by the jugal and prosquamosal. The quadratojugal is 

 relatively small, intercalated between the prosquamosal and the 

 quadrate, and wholly separated from the jugal. Paleohatteria 

 may possibly have a separated prosquamosal in the same posi- 

 tion and with the same relations, but this fact, if fact it is, is yet 

 to be determined. The Triassic Hyperodapedon and the allied 

 Stenomctopon (Fig. I 5) have no separated prosquamosal. SapJiceo- 

 sawus, from the Jurassic, more nearly ancestral, perhaps, to 

 the modern Sphenodon, in all probability possesses a separated 

 prosquamosal intercalated between the quadratojugal and the 

 jugal, as in Dimetrodon. Finally in the living Sphenodon, not- 

 withstanding its many primitive characters, the prosquamosal 

 has utterly disappeared, even in the embryo, according to Howes 

 and Swinnerton, though the squamosal is continued above the 

 quadratojugal to unite with the jugal (Fig. 6). Here the quad- 

 ratojugal articulates with the jugal, as in all the archosauria, the 

 intervening bone having disappeared. Baur 2 believed that the 

 prosquamosal was present in Sphenodon though early fused with 

 the squamosal, a view which now is shown to be incorrect. 

 The bone has been gone so long that it fails to make any im- 

 pression upon the embryo. 



From all of which evidence, Dimetrodon, Sapluzosanrus, pos- 

 sibly Paleohatteria, and the conditions now existing in Sphenodon, 

 the conclusion is irresistible that the laterotemporal vacuity was 

 originally formed in the true diapsid reptiles not below, but above 

 the prosquamosal ; that this bone has nothing to do with the 

 upper bar in these reptiles, which without exception was formed 

 primitively by the squamosal and postorbital ; secondarily by the 

 squamosal, postorbital and jugal ; finally in the theriodonts and 

 mammals by the squamosal and jugal alone. The prosquamosal 

 never forms the outer margin of the supratemporal fossa. 



All this leads us to the consideration of the arch in the Squa- 

 mata wherein the condition of things has caused no end of con- 



1 Journal of Geology. 



2 Anat. Anzeiger, X., p. 321, 1895. 



