OSTEOLOGY OF AMERICAN PERMIAN VERTEBRATES 399 



are due to Dr. Broili. I reached the decided conviction that 

 Acrosaurus is merely the young of Pleurosaurus. Of course the 

 presence of a single upper temporal vacuity excludes the form 

 decisively and absolutely from the Rhynchocephalia or Diapto- 

 sauria, even. Whether or not it is intimately allied to Araeoscelis 

 and Frotorosaurus I shall not presume to say; but certainly its 

 claims to such relations are vastly greater than those with the 

 rhynchocephalian forms. 



Proganosauria. — If Huene is correct in the attribution of a 

 single upper temporal vacuity to Mesosaurus (and I believe that 

 he is, notwithstanding the imperfect material that is in the collec- 

 tions of the skull of this genus), there may arise a question of the 

 relationships between it and some of the forms I have briefly dis- 

 cussed. So far as the skeleton of the proganosaurs is concerned, 

 it is simply an aquatic modification of the primitive cotylosaurian, 

 or better, pelycosaurian type. Here, also, until the more precise 

 structure of the skull is known, speculation is idle. The order 

 Proganosauria may be accepted until it is proved that it has no 

 claim for independent existence. 



Ichthyosauria. — The origin and relationships of the Ichthyo- 

 sauria have long been an unsolved problem, but little nearer 

 solution today than it was a score of years ago, notwithstanding 

 the brilliant discoveries of Dr. Merriam. On the strength of its 

 generalized characters it has been associated with the rhyncho- 

 cephalian type, as a member of the double-arched division of rep- 

 tiles, under the hypothesis that the lower vacuity was secondarily 

 closed by the encroachment of the orbit. More recently Huene 

 has urged that its relationships are nearest with the Proganosauria, 

 and with much reason. This much can be said: If all the known 

 reptiles having a distinctive upper temporal opening have arisen 

 from a single ancestral type, that is, are monophyletic, then the 

 ichthyosaurs must be more closely related to the Squamata, 

 Frotorosaurus ^ and Proganosauria, than to any other known rep- 

 tiles This much I believe : The Ichthyosauria are of very primi- 

 tive origin; they have never possessed a lower temporal vacuity; 

 the two bones of the temporal region, variously known as supra- 

 temporal and squamosal, point to a direct origin from the stego- 



