420 S. W. WILLISTON 



by their direct descendants today; the birds, crocodiles, and 

 tuatara of the Diapsida; the mammals of the Synapsida; the 

 lizards and snakes of the Parapsida; and the turtles of the Anap- 

 sida. They suggest the following linear arrangement of the 

 known groups, the doubtful or poorly known ones, perhaps entitled 

 to ordinal rank, printed in italics: 



Anapsida Diapsida 



Cotylosauria Rhynchocephalia 



Chelonia Rhynchosauria 



Synapsida Thalattosauria 



Theromorpha Choristodera 



Therapsida Phytosauria 



Sauropterygia Pseudosuchia 



Placodontia Crocodilia 



Parapsida Pterosauria 



Ichthyosauria Dinosauria 



Squamata "Eosuchia" 



Protorosauria (Araeos- 

 celidia, Acrosauria) 



I am aware that other general phylogenetic schemes of the 

 Reptilia have been proposed, especially by Boulenger and Goodrich, 

 but long years of study of the reptiles has convinced me that, 

 while all may have features worthy of consideration, the' chief 

 reliance must be placed upon the skull structure, especially that 

 of the cranial and temporal regions. As von Huene and others 

 have urged, these parts are the most conservative, and least liable 

 to homoplastic duplication. Next to the skull, the ribs are con- 

 servative. In all the Archosauria the double-headed dorsal ribs 

 are attached to the diapophyses. In the Diaptosauria (except 

 Thalattosauria), the Synapsida (except the Sauropterygia), and 

 Anapsida the rib tubercle articulates with the arch, the capitulum 

 with the intercentral space, while in the Parapsida, so far as known, 

 the ribs are attached more or less exclusively to the centra. Other 

 characters originally proposed as distinctive between the Diapsida 

 and the Synapsida in the wider sense have been proven to be invalid. 

 We know that the primitive foot structure is nearly that of the 

 lizards and Sphenodon of today, that the reduction of the phalanges 

 in the theriodonts and turtles is purely homoplastic. The supposed 

 relationship between the turtles and the plesiosaurs is also purely 



