1899.] THE MA.RSUPI.VL AND PLVCEXTAL CABXTVORA. 927 



Prothyl acinus, on the other hand, will have the formula : — 



mi. 1 : mi. 2 . mi. 3 . mi. 4 -iiic. 1 mp. 1 . pp. 2 . pp. 3 . mp. 4 m.l.m.2. m. 3 



mi. 1 . mi. 2 . mi. 3 /«c. 1 mp. 1 . /)jo. 2 . pp. 3 . mp. 4 ?». 1 . »*. 2 . m. 3 



.Finally, in Thi/lacinus we shall have : — 



mi. 1 . wj. 2 . «jz'. 3 . 7}ii. 4 »«'". 1 ?«;?. 1 . mp. 2 . pp. 3 . w^^ . 4 m. 1 . ?». 2 . ;». 3 

 wi. 1 . }?ii. 2 . ini. 3 ;Mr;. 1 /«/». 1 . mp. 2 . pp. 3 . »?/) . 4 m. 1 . ?«. 2 . m. 3' 



In ordinary practice, however, when the number, rather than 

 the snccessional homology, is the point to be elucidated, we may 

 follow a modification of the practice now employed. 



Hyccnodon will remain as before, viz. i. '^, c. ^, p. j, m. g ; and 



ProthylaGhms and Thi/lacinus will be indicated by i. 3, c j, hi/^ & 



p. ^, >H. ;^, Possibly an emendation may be necessary in regard to 

 the detailed formula of Hycenodon, for as the first cheek-tooth (as 

 in almost all other Placentals) is not replaced, it may really be a 

 persistent milk-premolar instead of a permanent premolar. Indeed 

 the condition occurring in Rhinoceros suggests that such is probably 

 the case. 



In conclusion, I may depart so far from the subject indicated 

 by the title of this paper as to express my opinion that the Prothy- 

 lac'midce (for I see no reason for regarding the " Sparassodonta " 

 as representing more than a single family) are undoubtedly Mar- 

 supials, and that they are not very far removed from the Dnsi/uridce, 

 of which they may represent the ancestral type. They also appear 

 to be related to the Creodontia, which are themselves in all proba- 

 bility the ancestors of both the modern Oarnivora and Insectivora. 

 The Creodonts, on this view, have retained a tooth-change which 

 is lost in the modern Marsupials ; and both groups may be derived 

 from Mesozoic ancestors like Triconoclon and Amphitlierium, in 

 which, as appears to be indicated in the first-named of these, there 

 must have been a complete tooth-change. Evidence of such an- 

 cestry is afforded by the retention in Myrmecohias of the numerous 

 true molars distinctiA^e of some of the Mesozoic genera ; while, as 

 an abnormality, four true molars may occur in other modern 

 Marsupials, such as Didelphys. If these Mesozoic mammals be 

 rightly regarded as the common ancestors of both Creodonts and 

 Dasyurids, it is more than doubtful if they can any longer be 

 classed as " Marsupials," setisu stricto, for, in addition to possessing 

 a complete tooth-change, it is, in the light of recent researches, 

 quite possible, if indeed not probable, that they may have also 

 been placentiferous. 



I may add that the nomenclature proposed for the teeth of the 

 Placental Carnivora will also be applicable to those of the other 

 Placental orders. 



60* 



