560 



SR. F. AMEGHINO ON THE 



[May 2, 



Diprotodonts. The latter are the descendants of the Diprotodonts 

 which in former times inhabited Argentina. The multitiibereulate 

 i-ondition of the fossil Diprotodonts of the Northern Hemisphere 

 is the outcome of the duplication of the molar ciisps of the Pauci- 

 tnberculata. The Cretaceous and Eocene fossil forms of Argentina 

 exhibit all the intermediate stages between the Multituberculata 

 and the Pancituberculata : amongst these there is one, the Man" 

 nndon, in which the molars show a complication of exactly the 

 same type as that presented by the classical molar of MicroJestes 

 anfiquus, figured in all the manuals of palaeontology. 



Eig. 5. 



Halviariphits guaraniticus: fifth right lower molar, superior (a), internal (h), 

 and external (c) aspect, eight times nat. size. — Uppei" Cietaceous ; Patagonia. 



In the molars of the Cretaceous Rodents of Argentina the 

 derivation from the sexcuspidate type is equally recognizable. The 

 Caviidae, \vith their molars formed of two triangular or cordiform 

 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 Didelphyidse. However, the numerous fossil forms 

 of this series graduate without interruption between the recent 

 Caviidse and the Eocene Eocaixlidfe, and between the latter and the 

 Cretaceous Cephalomyidae. Eig. 6 shows the seventh (ultimate) 



Eisf. 0. 



Cepholomys prorsus : last riglit lower molar, superior nspect, eight 

 times nat. size. — Upper Cretaceous ; Patagonia. 



right lower molar of Ceplmloinys prorsiis, 8 times nat. size. In 

 the two lobes of this tooth it is easy to recognize the two prisms 

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

 primitive cusps are likewise discernible, though disposed sHghtly 

 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. 



