368 Dr. W. Kiikentlial on the Origin and 



common ancestors. In any case the birds have nothing to 

 do with the order of the flying reptiles to which the remark- 

 able Pttrodactylus belongs. The origin of the Pterosauria is 

 as yet by no means elucidated. While the Chelonians are a 

 strongly specialized branch, which is perhaps to be derived 

 from a group of the Theromorpha, the Lacertilia have their 

 root in the primeval Rhynchocephala, From them there 

 branched off during the Cretaceous period the pelagic Pytho- 

 nomor])hn, which soon became extinct again, as also the 

 snakes, which are still in existence. 



Having thus given a brief outline of the phylogeny of the 

 Ke])tiles as it is now pretty generally accepted, we must now 

 proceed to inquire from which of their orders the phylum of 

 the Mammalia can have sprung. To this question an answer 

 has been given to the effect that the already-mentioned Thero- 

 morpha are regarded as ancestors of the Mammalia, since 

 they exhibit the greatest similarity to them. As a matter of 

 fact a comparison of the skeletons, according to which alone 

 we can proceed, since no other remains have come down to 

 us, exhibits a considerable number of similar characters in 

 the two groups ^. 



Especially striking is the oft- quoted resemblance in the 

 differentiation of the dentition. As among the Mammalia, so 

 also in the Theromorpha, we find a morphological difference 

 within the dental series ; here also we may speak of incisors, 

 canines, and molars, in contradistinction to other reptiles, in 

 which only uniformly conical teeth exist in the jaw. It 

 therefore appears to be imperative that we should undertake 

 a closer consideration of the Theromorphous dentition. 



Of the four suborders of the Theromorpha the greatest 

 number of resemblances in dentition to other reptiles is exhi- 

 bited by the Pareiasauria. In these creatures all the teeth, 

 tlie number of which was fairly large (seventy-six in Pareia- 

 saunfsbomMdens),weve devoted to tolerably similar functions, 

 and accordingly exhibit only small differences in their struc- 

 ture. In all the genera described by Owen [TapinocephaluSj 

 Fareiascairus, and Anthodon) distinct rudiments of succes- 

 sional teeth are present internally to the dental series. 



Far greater different iation is found in the dentition of the 

 Theriodontia, whose teeth are constructed according to the 

 carnivorous type. No trace of rudiments of successional 

 teeth has been found in any of these predaceous reptiles. 



The two other suborders have a dentition which is very 



* Co\<e, " The Relations between tlie Theromorphous Reptiles and the 

 Monotrema Mammalia," Proc. Amer. Assoc, for the Advancement of 

 Science, vol. xxxiii. (1885). 



