184 



MR. J. M. S. WATSON ON 



the Labyrinthodontia and Lepospondylia, but the resemblance 

 may easily be due to inheritance from common ancestors amongst 

 the Crossopterygian fish. 



The ordinal division of the Labyrinthodontia marks three 

 stages in the evolution of that group. It is founded primarily 

 on the skull, which becomes more and more depressed, the basi- 

 occipital and basisphenoid being gradually reduced from the quite 

 large bones they are in Pteroplax to the small slightly ossified 

 tracts lying on the parasphenoid which represent them in 

 Stereospondyls. The development of the vertebral column is 

 used as a check on this classification. 



Notes on the Reptilian Classification. 



The super-order Cotylosauria is retained simply because of its 

 use as a dumping-ground for those primitive reptiles which retain 

 a roofed skull. The orders are also probably somewhat unnatural 

 groups, and it would perhaps have been more satisfactory to raise 

 the super-families to ordinal rank. 



The real classification, i. e. that into families, is founded as far 

 as possible on the characters of the brain-case, which have been 

 discussed by me in a series of papers. 



The separation of the super-order Anomodontia is founded on 

 its very characteristic brain-case, and for the typical forms on the 

 equally characteristic lower jaw. 



The remarkable animal Casea is obviously ordinally different 

 from all other well-known forms, its position in the super-order 

 Anomodontia is very doubtful, but it is not improbable that it 

 represents a very early offshoot from this stock. 



The order Pelycosauria is referred to the super-order Anomo- 

 dontia with certainty ; in Dimetrodon the brain-case and lower 

 jaw are absolutely typical, and the other forms included are 

 obviously allied to this type by the whole of their structure. 



The super-family division is founded on differences in the 

 stapedial articulation with the brain-case, which in the three 

 suborders seem to form a morphological series. 



The order seems to be self-contained, culminating in Dime- 

 trodon and Edcqihosaurus, towards which the other types lead. 

 At the same time the trend of evolutionary change in the brain- 

 case is the same as that which, continued in later times by the 

 South- African forms, leads up to the mammals. 



The super-family Bolosauridpe is founded for very badly known 

 reptiles, which it might perhaps have been wiser to leave as 

 Anomodontia incerta sedis. The position of the super-family in 

 the super-order depends mainly on the badly preserved lower 

 jaw. 



The order Deinocephalia is certainly a member of the super- 

 order, having the typical brain-case and lower jaw. It is distin- 

 guished from the Pelycosaurs by its advanced limbs, shown most 



