H. F. O shorn — Origin of Mammals. 95 



Always keeping in mind that our direct evidence here is of 

 the most limited character, since we have neither skulls nor 

 skeletons, only teeth and jaws, we are tempted to hypothetically 

 connect the Creodonta with the Insectivora Primitiva, and to 

 assume that there existed in the Mid-Jurassic, as above stated, 

 the well advanced radiation Y of the two sub-classes of Eutheria 

 and Prototheria as defined by Gill. 



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. Pareiasanria or Cotylosauria, land animals with a solid 

 skull and replete with Stegocephalian or Amphibian characters, 

 certainly the most primitive reptiles. 



2. Proterosauria or Proganosaaria, with an open two-arched 

 skull, specialized reptiles, which have apparent affinities with 

 the Crocodilia, Dinosauria, Rhyncocephalia and Squamata (Doli- 

 chosauria, Mosasauria, Lacertilia and Ophidia). 



3. Theriodontia or Theromora (Dicynodontia, Cynodontia and 

 Gomphodontia), with an open, single-arched, skull (as in the 

 Ohelonia, Plesiosauria, Icthyosauria and Mammalia). 



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, terres- 

 trial 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 Monotreme 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-tri- 

 tubercular or multitubercular in form. The skeleton so far as 

 known is partly Eutherian, partly Monotreme or Prototherian. 



Two alternatives at this point present themselves : First : Are 

 the Monotremes directly derived from such a type as these 

 Theriodonts, leaving an independent derivation for the Euther- 

 ian or Marsupio-placental stock, as has been suggested by 

 Seeley? Second: Must we set aside the Theriodonts for an 

 Amphibian stem form on account of the numerous Amphibian 

 resemblances which Hubrecht and others find in the develop- 

 ment and anatomy of the lower Mammalia? 



