5G4 



THE AMEBIC AX XATURALIST [Vol.XLVI 



Williston evidently regards' the presence of a temporal vacuity 

 as sufficient to justify the separation of Varanosaurus and Casea 

 from the Cotylosauria ; and he may be right. I lis position could 

 not be questioned if it could be shown that the presence or the 

 absence of this feature indicated the divergence of two phyla; 

 that the one group gave origin to descendants that retained the 

 temporal roof intact, while the other started a line that developed 

 one or two vacuities on each side. However, that proposition 

 can hardly be proved as yet. 



In Varanosaurus the temporal roof is mostly lacking and there 

 is no lower temporal arch, differing in the latter respect greatly 

 from Casea, Dr. Williston is led to discuss the value of the 

 vacuities and arches in the classification of the reptiles. He 

 recognizes three chief types, perhaps three chief phyla: (1) the 

 Cotylosauria, with unbroken temporal roof; (2) the type in 

 which there are two vacuities and two arches; (3) the single- 

 arched type, in which there is a single vacuity bounded below 

 by the jugal and quadratojugal. He thinks that there may be 

 a fourth type, that in which a vacuity is bounded below by the 

 postorbital and the squamosal. He is, however, unable to see the 

 distinction between the two types with a single vacuity, and is 

 inclined to believe that all single-arched reptiles have arisen from 

 a single type. The present writer is unable to understand clearly 

 the position taken. 



Inasmuch as the temporal roof is primitively, as in the Coty- 

 losauria, complete and composed of two series of bones, it is the 

 vacuities which developed in them that are the important matters 

 to consider. It seems to the writer that a single vacuity may 

 have originated in five different ways : 



1. By the development of the upper vacuity alone. 



2. By the development of the lower one alone. 



3. By the appearance and extension of a vacuity in the 

 postorbito-squamosal arch. 



4. By the gradual reduction of the postorbito-squamosal bar, 

 allowing the upper and the lower vacuities to unite. 



5. By the reduction of the lower arch, leaving only the upper 

 vacuity. 



The matter may be further complicated by changes in the 

 temporal roof such as are found in some of the turtles: (D I* s 

 lower border may be eaten away, resulting finally in a condition 

 such as appears to exist in Varanosaurus: (2) the hinder border 



