82 MUSEO NACIONAL DE BUENOS AERES. 



« la tlie moliirs of tlie Cretaceous Rodents of Argentina the de- 

 rivatiou from the sexcuspidate type is equally recognizable. The 

 Caviidae, with their molars formed of two triangular or cordiform 

 prisnis, and with an open cavity at the base, are those wich depart 

 most from the primitive form: it seems absolutely impossible to 

 make out iu these mokirs anj'thing approaehing these of the Di- 

 delphyidae. However, the uumerous fossil foi'ms of this series gra- 

 dúate without interruption between tlie recent Caviidae and the 

 Eocene Eocardidae, and between the latter and the Cretaceous 

 Cephalomjüdae. Fig. 6 (fig. 1) shows the seventh (ultimate) right 



Fig. 1. Cephalomt/s prorsim Amgh. Ultima muela inferior del lado derecho, vis- 

 ta de arriba, aumentada ocho diámetros (Jj del tamaño natural, ae, dentículo 

 anterior externo; ma, el mediano anterior; ai, el anterior interno; pe, el posterior 

 externo; pi, el posterior interno; mp, el mediano posterior. Cretáceo superior de 

 Patagonia (pyrotheriense). Reproducida de los Proeeed. of the Zoo!. Soc. of hon- 

 dón, 1899, p. 560. 



lower molar of Cephalomys prorsus, 8 times nat. size. In the two 

 lo bes of this tooth it is easy to reeognize the two prisms of the Ca- 

 viidae; but the six elements corresponding to the six primitive 

 cusps are likewisse discernible, though disposed slightly different- 

 ly from the ordinary. The three cusps of each lobe are disposed in 

 a triangle, the two esternal, ae, pe, maintaining their position; but 

 the two median, the anterior ma and posterior mp, are limited to 

 the internal margin. A somewhat similar disposition is seen in the 

 molars of some Cretaceous Diprotodonts of the family Garzonidae, 

 e. g. the molar of Halmarhíphus guaraniticus, rejjresented in fig. 5 

 (fig. 2). This agreement in the disposition of the primitive molar 

 elemeuts seems to imply that the Rodents, the origin of which is 

 still a mysterj', may represent a side branch of the Diprotodonts, 

 which originated towards the middle of the Cretaceous period » . . 

 (Ameghino, 12, p. 560-561). 



Poco tiempo después, Carlos Ameghiuo descubría en el cretáceo 

 superior de Patagonia, los primeros vestigios de esos antiguos di- 



