41 8 APPENDIX C. 



Creodonta with the Insectivora Primitiva, and to assume that there 

 existed, as above stated, a well advanced radiation (V) of the two 

 sub-classes of EUTHERIA and Prototheria. 



The problem of the Origin of the Mammals now resolves itself 

 into the connections between these two sub-classes, either with the 

 Reptilia or Amphibia, and we turn back to the three contemporary 

 upper Permian reptile groups : 



1. Pareiasauria or Cotylosauria, land animals with a solid 



skull and replete with Stegocephalian or Amphibian 

 characters, certainly the most primitive reptiles. 



2. Proterosaiiria or Proganosauria, with an open two-arched 



skull, specialized reptiles, which have apparent affinities 

 with the Crocodilia, Dinosauria, Rhyncocephalia and 

 Squamata (Dolichosauria, Mosasauria, Lacertilia and 

 Ophidia). 



3. Theriodontia or TJieroviora {Dicynodontia, Cynodontia and 



Goniphodoutia), with an open single arched skull (as in 

 the Chelonia, Plesiosauria and Ichthyosauria and Mam- 

 malia). 



The Theriodontia as perceived by Owen in 1876, and now fully 

 confirmed by Seeley, are astoundingly mammalian in type. They 

 are essentially quadrupedal, long-limbed, terrestrial reptiles, totally 

 dissimilar from all other reptiles, and standing far away from them. 

 They also present an advanced stage of functional radiation in 

 tooth and skull structure in adaptation to carnivorous, omnivorous, 

 and herbivorous habits, to which when known, their skeletons will 

 probably be found to conform. The shoulder girdle is of Mono- 

 treme type, in other features of the skeleton they are strikingly 

 like the ancestral Eutherian, described above. The skull may in 

 fact be reduced to the Eutherian type by the coalescence of the 

 prefrontals, postorbito-frontals, and quadrates with the adjacent 

 elements, and by the loss of the ectopterygoids or transverse bones. 

 The teeth are promammalian in formula, and pro-tritubercular or 

 multitubercular in form. The skeleton so far as known is partly 

 Eutherian, partly Monotreme or Prototherian. 



Two questions at this point present themselves : first, Are the 

 Monotremes directly derived from such a type as these Theriodonts, 

 leaving an independent derivation for the Eutherian or Marsupio- 

 placental stock, as has been suggested by Seeley .'' Second, Must 

 we set aside the Theriodonts on account of the numerous Amphibian 

 resemblances, which Hubrecht and others find in the development 

 and anatomy of the lower Mammalia } 



The first question is now unanswerable, although at present the 

 evidence that the Mammalia are diphyletic is certainly considerable. 

 Right here the investigation of the placental ovum rises to extreme 

 importance, if the Eutherian ovum is of Amphibian type, the Mam- 

 malia are certainly diphyletic, for the Monotreme ovum is certainly 



