644 S. W. WILLISTON 



we could distribute them at our pleasure. As a rule in taxonomy 

 we depend not so much upon unique distinctive characters as 

 we do upon different assemblages of characters common to various 

 groups. A half dozen characters, if constantly associated in 

 any given group would serve for its delimitation, the rank of 

 the group being dependent upon the relative importance of the 

 characters themselves. And the taxonomist is suspicious of 

 any generic, family or ordinal classification based upon single 

 characters. Rightly interpreted, such single characters, unaccom- 

 panied by other differences, may be and often are merely muta- 

 tions or sports, unstable and of little significance. From which 

 remarks it follows that I have no faith in the 'Mutation theory' 

 of the origin of species, nor do I believe that any paleontologist 

 can defend such a theory. And, frankly, neither do I believe 

 that any theory of the origin of species, or of evolution even, 

 can ever get very far when time is left out of account. If, in 

 any series of phylogenetic forms we find a gradual transformation 

 of structure, the gradual acquisition of new characters, we do 

 right in uniting them all in a single group, for the sole end of 

 all taxonomy is phylogeny. 



Let us now attempt to define the two larger groups, the Cotylo-. 

 sauria and Theromorpha, in order to discover their distinctive 

 or associated characters. Neither order has any single character 

 to distinguish it from all other reptiles. The non-perf orate tem- 

 poral roof is shared between the Cotylosauria and Chelonia; 

 the single temporal vacuity is common to both Casea or Edapho- 



sake, one is as good as another. Huene, Case and I call the small bone on the 

 posterior angle of the skull in the Captorhinidae and Procolophonia the tabulare 

 ('epiotic'); others call it the supratemporal, and no one knows which it is. I 

 at one time suggested the possible identity of the posterior arcade bone in the 

 lizards with the tabulare, but no one seems to approve of it; and yet the small 

 bone in the same position, and with like arrangements in the Procolophonia, Huene 

 accepts as the tabulare. How little we really know of the homologies is evident 

 enough in the fact that so acute an observer as Huene admits the possibility of 

 the persistence of either supratemporal or tabulare. For the sake of uniformity 

 solely, without committing myself to any theory of homology, I accept and use 

 the terms supratemporal for the upper, squamosal for the lower element, wherever 

 they occur, as in the Ichthyosauria, Lacertilia, Stegocephalia, Cotylosauria, 

 etc. 



