288 On the 5/rM?Z 0/ Tritylodon longtevu?, Otoen. 



of this last statement I add here in PI. X. fig. 3 a photograph 

 of two newly prepared molar teeth of Diademodon browni 

 with undivided roots*, and in PI. X. fig. 5 a photograph of 

 the newly prepared molar tooth of Cynognathus crateronoius, 

 sliowing undivided root and a groove on tlie outer surface, 

 indicating the beginning of a partial division of this root, as it 

 seems, throughout its whole length. 



II. Reptilian Characters. 



1. Divided nares; 



2. Prefrontal bone f; 



3. Frontal bone not bounding the orbit. 



III. Characters loth Mammalian and Reptilian. 



1. Septomaxillary bones; 



2. The terminal position of the anterior nares ; 



3. The backward position of the posterior nares; 



4. Considerably diverging parietal bones ; 



5. Orbito- or alispiieuoid (or orbitopalatiue ?) ; 



6. No postorbital bar; 



7. Brain-case antero-iaterally closed. 



If we try to separate in this last group the characters that 

 are primarily reptilian in TrityJodon from those wliich are 

 probably purely mammalian, we should regard only the 

 existence of the septomaxillary bone as a pure reptilian 

 cluiracter (common botii to Trityludon and to Monotremes). 

 So that Trilylodon is to be regarded, in the first place, as a 

 mammal, but as a mammal with some undoubtedly reptilian 

 characters, the most primitiveoi the known Mammals. With 

 its mixed characteis, Tritylodon is a direct proof that the 

 Mammals have their origin in Keptiles, most probably iji 

 theriodout Reptiles. 



* Seeley mentions (comp. " On Diademodoii " in Pliilos. Trans. 1895, 

 pp. 1029-1041, figs. 5-lOj three isolated molar teeth of Uiademodon with 

 imdivided roots, but they seem to belong to lower molars. 1 add here 

 in PL X. tig. 4 another photograph of two single-ruoted V-shaped teeth of 

 Diudemodun in section from the specimen E. ciOOo in the Briti»li Museum 

 (mentioned by Mr. Watson, " The Skull oi. Diademodon,'^ in Ann. & Mag. 

 Nat. Hist. (8) viii. 1911, p. 314), one with open and the other with 

 closed pulpa. 



+ The existence of a prefrontal bone in mammals has been aiSrmed 

 (comp. R. Cunningham, " Note on the Presence of Supernumerary Bonea 

 occupying the Place of Prefrontals in the Skull of certain Alammala," in 

 Proc.'Zool. Soc. Lond. 18b9, pp. 76-77). 



