IN THE KEPTILIA. 25. 



The Theromora are the earliest in time, and their order of Theriodonta presents 

 the nearest affinities to the Mammalia. The Cotylosauria, on the other hand, dis- 

 play the nearest relations to the Stegocephalous Batrachia. 



In the following illustrations the phylogenetic successions are indicated by dia- 

 grams based on the skull of Pantylus cordatus. This is adopted as the most conve- 

 nient of the Cotylosaurian types to be taken as a standard, because it displays none 

 of the especial peculiarities that characterize Chilonyx, and is better known in the 

 lateral posterior region than Pariotichus. In the series which terminates in the 

 iStreptostylica we commence with the Theriodont type in Fig. 1, with an infratem- 

 poral foramen only, and reach Diopeus or Sphenodon with both infratemporal and 

 supratemporal. This is naturally followed by Dicynodon with supratemporal 

 foramen and no zygomatic arch, from which we pass to the Lacertilia, which has the 

 ii'ee OS quadratum. The descent of the Lacertilia is fi'om the Theriodonta through 

 the Rhynchocephalia, the Anomodontia being a lateral branch. The Archosaurian 

 line may commence with a form with a supramastoid foramen only, or one with an 

 infra mastoid only. TV^e know no type of the latter kind ; and of the former we have 

 the aquatic Ichthyopterygia. A terrestrial type of this order probably existed, which 

 represents the stock from which the Archosaurian line is derived. In this series the 

 order of development probably has been Crocodilia, Dinosauria, Pterosauria, as 

 represented in Figs. 9 and 10, PI. Y. 



EXPLANATION" OF PLATES. 

 JPlate I. 



Stegocephali and Cotylosauria. 



Fig. 1. Mastodonsaiirus giganteus Jaeger, |- nat. size ; from Fraas. 



2. Chilonyx rapidens Cope, f nat. size; from above; premaxlllary, maxillary, jugal and zygomatic regions 



restored ; a, left side. Coll. E. D. Cope. 



3. Pariotichus megalops Cope, | nat. size ; zygomatic region imperfect ; from above ; a. left side. Coll. 



E. D. Cope. 



4. Pantylm cordatus Cope, | nat. size ; occipital region imperfect ; from above ; a, right side. The line pass- 



ing through the postorbital, postfrontal and supratemporal is a fracture of the osseous roof. Coll. 

 E. D. Cope. 



Plate IL 



Theriodonta and Rhtnchocephalia. 



Fig. 5. Edaphosauras pogonias Cope, | nat. size ; with imperfect zygomatic region, seen obliquely from above ; 

 a, posterior view. Coll. E. D. Cope. 

 6. Clepsydrops natalis Cope, J nat. size ; with superior posterior part of skull imperfect ; left side. Coll. 

 E. D. Cope. 



A. P. S. — VOL. XVII. D. 



