THE TEMPORAL ARCHES OF THE REPTILIA. 183 



Since the above was written, I have received the following 

 statement from Dr. Broom, kindly sent in reply to a query 

 from me : " No trace of either prosquamosal or quadratojugal has 

 been found in any Anomodont or Theriodont. The large bone 

 which supports the quadrate in Dicynodon and also forms so 

 large a part of the temporal arch is all pure squamosal. The 

 same is the case in the Therocephalia and Theriodontia. The 

 upper part of the quadrate which rests on the squamosal has been 

 thought by some to be the quadratojugal, but I am pretty con- 

 fident that the whole is quadrate, and that no rudiment even of 

 the quadratojugal is present." 



From these facts we may conclude that the emargination of 

 the single bar has removed all of the lower arch, and that all 

 which remains corresponds to the upper arch of the diapsid rep- 

 tiles, the squamoso-postorbito-jugal bar. Certainly until some 

 form is discovered in which the prosquamosal and quadratojugal 

 are definitely shown to be present as a part of the arch, the com- 

 pound nature of the arch in the synapsid forms is hypothetical. 

 One species of Cynognathus has been figured (Fig. 14) with a 

 small fenestra between the squamosal and jugal. No other 

 species of this genus possesses this opening, and its presence in 

 this species, is, I believe, disputed by Dr. Broom. Even if it be 

 present, it has no classificatory significance, since it must have been 

 a parallel development in these animals ; an abortive attempt, as it 

 were, without phylogenetic significance. It is of interest, perhaps, 

 if it really occurs in any of the anomodonts, as showing the ten- 

 dency to perforation of a broad plate covered on both sides by 

 muscles. 



The question is properly asked : How has the mammalian 

 zygoma arisen ? The answer does not seem doubtful to me. 

 The mammalian arch has the structure of that of LycosucJius (Fig. 

 12), and is composed of the squamosal and jugal alone. This it 

 seems to me, will be made apparent by the consideration of the 

 arch in Placodus (Fig. 13), Cynognatlius (Fig. 14), and Lystro- 

 saurus (Fig. 2), in which the articulation of the squamosal is 

 with both the postorbital and the jugal in about equal measure. 1 



1 " I can find no difference in the character of the arch in the Anomodontia and 

 Theriodontia except that in the Theriodont the malar bone has a greater external back- 



