24 



POSTERIOR CRANIAL ARCHES 



the Testudinata, where both sets of elements are included in a single bar. This com- 

 pound bar is, however, reduced to the zygomatic elements in the more specialized 

 forms, and is not unfrequently entirely lost. 



Aided by these considerations we get the following phylogenetic series. Each 

 one of them originated in the Permian epoch. This table resembles essentially the 

 one I gave in the article on the Evolution of the Vertebrata in the American Natu- 

 ralist for 1885, p. 247, in which all the later orders were traced to the Theromora, 

 the Lacertilian series through the Rhynchocephalia. The varied character of that 

 assemblage was not at that time suspected, but it is true that there is a great 

 resemblance between the orders now included in it, except in the matter of the 

 cranial roof and bars, and in the nature of the rib-articulations. The discovery that 

 Diopeus is allied to the Rhynchocephalia places that order in immediate relation with 

 the Theromorous series on the one hand ; while a correct estimate of its cranial 

 structure places it in immediate relation with the Lacertilia on the other. 



Recent. 

 Rhynchocephalia. Crocodilia. Lacertilia. Ophidia. Testudinata. 



Lacertilia. 



\ 



/ 



Pterosauria. 



Dinosauria. 

 Mbsozoic. ^ 



Crocodilia. 



Ichthyosauria. Sauropterygia. 



Paleozoic. 



Lacertilia. Ophidia. 



Pythonomorpha. 



Testudinata. 



Anomodonta. 



Rhvnchocephalia. 



I 

 Theriodonta. 



Cotylosauria. 

 The five reptilian series might be then further defined as follows : 



Quadrate fixed ; no supramastoid or supratemporal foramen or separate arch Tlicromora. 



Quadrate fixed ; a supramastoid foramen and arch IchtJiyopterygia. 



Quadrate fixed ; a supratemporal and zygomatic arch Archosauria. 



Quadrate fixed ; a zj-gomatic arch only Synaptosaurza. 



Quadrate free ; no supramastoid foramen or arch ; a supratemporal but no zygomatic arch. 



Streptostylica. 



