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Show 1899.] THE MARSUPIAL AND PLACENTAL CARNIVORA. 927 Prothylacinus, on the other hand, will have the formula:- mi. 1 . mi. 2 . mi. 3 . mi. 4 me. 1 nip. 1 . pp. 2 . pp. 3 . mp. 4 m. 1 . m. mi. 1. mi. 2 . mi. 3 ~ mc. 1 mp. 1 . pp. 2 . pp. 3 . mp. 4 m. 1. m. 2 . m. Finally, in Thylacinus we shall have :- mi. 1 . mi. 2 . mi. 3 . mi. 4 mc. 1 mp. 1. mp. 2 . pp. 3 . mp . 4 m. 1. m. 2 mi. 1 . mi. 2 . mi. 3 ~mc. 1 mp. 1. mp. 2 . J»/J. 3 . mp .4 m.l.m.2. m. In ordinary practice, however, when the number, rather than the successional homology, is the point to be elucidated, we may follow a modification of the practice now employed. Hycenodon will remain as before, viz. i. '^, c. v p. ±, m.^; and Prothylacinus and Thylacinus will be indicated by i. ^, c -, mp. & p. -., m. s. Possibly an emendation may be necessary in regard to the detailed formula of Hyatnodon, 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 m y opinion that the Prothy-lacinidce (for I see no reason for regarding the " Sparassodonta " as representing more than a single family) are undoubtedly Marsupials, and that they are not very far removed from the Dasyuridce, of which they may represent the ancestral type. They also appear to be related to the Creodontia, which are themselves in all probability the ancestors of both the modern Carnivora 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 Triconodon and Amphitherium, in which, as appears to be indicated in the first-named of these, there must have been a complete tooth-change. Evidence of such ancestry is afforded by the retention in Myrmecobius of the numerous true molars distinctive 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," sensu 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 place ntiferous. 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* |