PHYLOGENY AND CLASSIFICATION OF REPTILES 415 



simple separation in reptiles with a lower temporal vacuity of 

 the orbito-squamosal arch, leaving an upper temporal opening. 

 The bones here are usually loosely connected, so loosely indeed in 

 Dimetrodon that it is only within the past few years that we have 

 become sure that the separation was not permanent; a permanent 

 separation, one that would admit a knife blade even, and the deed 

 is done. Whether or not Ophiacodon is a real example of this 

 beginning, and I believe that it is, from such forms as Mycterosaurus 

 the step to the diapsid type is trivial. If I am correct, and I am 

 confident that I am, we then have the origin of the double-arched 

 reptiles in early or middle Permian times from theromorph reptiles 

 with a single, typically lower, temporal opening, possibly from forms 

 not unlike Paleohatteria. 



This group or subclass, which, with due modifications of the 

 original concept, may properly bear the name Synapsida given to it 

 by Osborn, includes scores of well-known genera of the orders 

 Theromorpha, Therapsida, and doubtless also the Sauropterygia. 

 It is the group that gave origin to the mammals, and has long 

 since been extinct. In its simplest and most primitive types, 

 together with all primitive characters of the skeleton, it has a 

 single temporal opening bounded below by the jugal, above by the 

 postorbital and squamosal. This opening, I believe, arose by the 

 separation of the squamosal and jugal, and not by a definite 

 perforation of any bone. And this opening is the sole char- 

 acter by which the group is ultimately distinguished from the 

 Cotylosauria, its ancestral stock. 



The evolution of the theromorph type through the dinoce- 

 phalian, therocephalian, and theriodont to the mammalian seems 

 assured by the African discoveries. In this evolution the structure 

 of the temporal region has undergone changes of which we do not 

 yet feel sure. The theriodont and anomodont reptiles, like the 

 plesiosaurian, may have the temporal opening extending from 

 the jugal to the parietal, apparently homologous with the combined 

 openings of the diapsid forms, or, as Watson has suggested, with 

 neither the synapsid nor diapsid ; but, tracing the development as 

 a whole, I should sooner believe that they all arose from the prim- 

 itive type like that of Dimetrodon or Mycterosaurus, that is, from 



