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Show 560 SB. F. AMEGHINO ON THE [May 2, Diprotodonts. The latter are the descendants of the Diprotodonts which in former times inhabited Argentina. The multituberculate condition of the fossil Diprotodonts of the Northern Hemisphere is the outcome of the duplication of the molar cusps of the Pauci-tuberculata. The Cretaceous and Eocene fossil forms of Argentina exhibit all the intermediate stages between tbe Multituberculata and the Paucituberculata ; amongst these there is one, the Man-nodon, in which the molars show a complication of exactly the same type as that presented by the classical molar of Microlestes antiquus, figured in all the manuals of palaeontology. Fig. 5. Halmariphus yuaranitieus: fifth right lower molar, superior (a), internal (b), and external (c) aspect, eight times nat. size.-Upper Cretaceous ; Patagonia. In the molars of the Cretaceous Eodents of Argentina the derivation from the sexcuspidate type is equally recognizable. The Caviidae, with their molars formed of two triangular or cordiform prisms, and with an open cavity at the base, are those which depart most from the primitive form : it seems absolutely impossible to make out in these molars anything approaching those of the Didelphyidae. However, the numerous fossil forms of this series graduate without interruption between the recent Caviidae and the Eocene Eocardid&i, and between the latter and the Cretaceous Cephalomyidar;. Fig. 6 shows the seventh (ultimate) Cephalomys prorsus : last right lower molar, superior aspect, eight times nat. size.-Upper Cretaceous; Patagonia. ri<_-ht lower molar of Cephalomys prorsus, 8 times nat. size. In tbe two lobes of this tooth it is easy to recognize the two prisms of the Caviidae ; but the six elements corresponding to the six primitive cusps are likewise discernible, though disposed slightly differently from the ordinary. The three cusps of each lobe are disposed in a triangle, the two external, ae, pe, maintaining their position ; but the two median, the anterior ma and posterior mp, |