1899.] 



PLEXOUOXT ilOLAES OF MAMMALS. 



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three cusps of the posterior lobe are the median-posterior, the 

 postero-external, and the postero-internal. 



According to the theory of gradual complication, the molars 

 provided with these six cusps ought to belong to the most recent of 

 Mammals. I shall show, however, that in all Mammalia, with the 

 exception of the Mouotremes, the Edentates, and the Cetaceans, 

 the plexodont type is the most ancient and the starting-point of 

 the different forms of complicated molars. 



For the demonstration of the antiquity of this type, I shall 

 avail myself of the numerous palseontological materials which the 

 Cretaceous and Lower Tertiary deposits of Argentina have yielded. 



The oldest fossil Mammalia of Argentina come from the varie- 

 gated sandstones which in Patagonia underlie the G-uaranian 

 formation with gigantic Dinosaurians. The best-known genus, 

 recently discovered, is the Proteodi'hlpJiJ/s prcpnirsor, the mandible 

 of which,, four times enlarged, is represented in fig. 1. Eodidelp^ys 



Profeodidelphys pracuTSor : right nianclibulav ramus, outer aspect, four 

 times nat. size. — Lower Cretaceous ; Patagonia. 



and Microhioiherivm. of the Upper Cretaceous and Eocene, connect 

 the former genus with the recent Didelphyidse, so that Proteo- 

 diclelphi/s represents the most ancient stem of this group ; it has 

 at the same time many aifinities with Pmtrodon of Marsh, and 

 other allied genera from the Upper Jurassic of North America. 



Eig. 2 represents the sixth lower molar, right side, of Proteo- 

 didelphys — a, outer view, 6, upper view (magnif. 8 times). It may 

 easily be seen that this tooth is composed of the two lobes and 

 the six cusps before mentioned, which I designate by the following 

 letters, the names in parentheses being those of Osborn's nomen- 

 clature. 



'ma, aiedian-anterior (paracouid). 

 ae, antero-esterual (protocouid). 

 ui, autero-internal (luetacoiiid). 



-pe, postero-extenial (bypoconid). 



pi, postero-internal (entoconid). 



mp, median -posterior ^ (hypoconulid). 



' This cusp is generally diminutive and in the small forms to be seen only 

 with the help of a strong lens. It loses its independence at an early date, by 

 bpcomiug fused either with cusp/ii or with cusp pe, ihe latter occurrence being 

 the more frequent. 



