OF THE PELYCOSAUKIAN GENUS DIMETRODON. 9 



a single arch {Si/)iapsida) for the possible affinity of the Pehjcosauria and Therocephalia, 

 the last the acknowledged ancestors of the Tlteriodonis, shows that the ancestors of the 

 two groups may have been common and have had two arches, at least potentially. 



The determination of the composition of the temporal arches and the identifica- 

 tion of the foramen quadratum in the Pehjcosauria enables certain comparisons to be 

 made that shed some light on the possiljle history of the development of the temporal 

 region in general. 13aur has claimed that the squamosal of Sphenodon is the united 

 prosquamosal and squamosal of the Lacertilia and has cited the condition of Saphsco- 

 saxti-us to prove this ; on the other band the evidence of embryology is negative or 

 even against this idea, for Howse and Swinnerton have shown that there is but a single 

 center of ossification in the developing squamosal of Sphenodon ('93), a fact admitted 

 by Baur ('94), and Parker has shown that there is but a single center of ossification for 

 the squamosal of the Crocodilia. 



In the specimens of Dimctrodon here described we have the most perfect example 

 of the skull of the primitive Archosauria {= Diaptosaiiria, Osborn) known ; it is unfor- 

 tunate that the specimens should be of the most specialized members of the group but 

 a comparison with a less perfect skull of a more generalized member of the same 

 family, IHopeus (Case, '03') shows that the primitive condition has remained largely 

 unaffected by minor changes. As shown in the figures, the prosquamosal of the Pely- 

 cosauria occupies the position of the quadra to-jugal in higher forms, i. e., it connects 

 the jugal and the quadrate region ; it articulates with the postorbital above and the 

 quadrato-jugal Vjehind, and is separated from the squamosal b}^ the union of the quad- 

 rato-jugal and the parietal. It, is evident that the shortening of the quadrato-jugal 

 and its withdrawal from contact with the parietal would permit the meeting and 

 possible union of the squamosal and prosquamosal ; if the two bones united it w^ould 

 produce the exact condition of the skull of Spjlicnodon, for all the other bones have the 

 same relations in the two forms and the Sphenodon has a forward prolongation of the 

 squamosal which is exactly the same in form and relations as the separate pro- 

 squamosal of the Pehjcosauria. This with the separate condition of the two bones 

 in Saphfi'osaurus and in the Icthyosauria would seem to establish the primitive freedom 

 of the bones beyond question were it not for the antagonistic embryological evidence ; 

 because of this it seems best to present the case in full. 



Concerning the region, Baur said ('94, p. 321) : " Es handelt sich nun darum, zu 

 zeigen, dass das squamosum von Sphenodon in der That aus 2 Elementen besteht. 

 Der jiingste von 6 schjideln, den ith vor niir hal)e (Condylis-occipitalis-Prtemax, 25 

 mm.) zeigt keine andeutung von 2 elementen ; dagegen scheint bei Saphaeosaurus 

 (Sauranodon) aus dem lithographischen Schiefer von Cirin das squamosum durch 2 



