560 



BR. F. AMEGHIXO OX THE 



[May 2, 



Diprotodonts. The latter are the descendants of the Diprotodonts 

 whith in former times inhabited Argentina. The multitiibereulate 

 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 the Multituberculata 

 and the Paiicituberculata : amongst these there is one, the Man- 

 nodon, in which the molnrs show a complication of exactly the 

 same type as that presented by the classical molar of Mia-ohstes 

 antJqmis, figured in all the manuals of palaeontology. 



ae- 



Halmariphus guarajiili'tis: fifth right lower molar, superior {n). internal (A), 

 and estemal {c) aspect, eight times uat. size. — Upper Cretaceous ; Palagonia. 



In the molars of the Cretaceous Eodents of Argentina the 

 derivation from the sexcuspidate type is equally recognizable. The 

 Caviidse, with their molars formed of two triangular or cordiforni 

 prisms, and with an open cavity at the base, are those which 

 depart mo^t from the primitive form : it seems absolutely 

 impossible to make out in these molars anything approaching 

 those of the Didelphyidae. However, the numerous fussil forms 

 of this series graduate without interruption between the recent 

 Caviidse and the Eocene Eocardida-, and between the latter and the 

 Cretaceous Cephalomyidae. Fig. 6 sliows the seventh (ultimate) 



Fig. 6. 



CephaloiMfK prorsui' : last right lower molar, superior aspect, eight 

 times nat. size. — Upper Cretaceous; Patagonia. 



right lower molar of Cephftlomys prorsus, 8 times nat. size. In 

 the two lobes of this tooth it is ea.«y to recognize the two prisms 

 of the Caviidaj ; but the six elements corresponding to the six 

 primitive cusps are likewise discernible, though disposed sliglitly 

 differently from the ordinary. The three cusps of each lobe are 

 disposed in a triangle, tlie two external, ae, pc, maintaining their 

 position ; but the two median, the anterior ma and posterior mp, 



