1809.] 



rr.EXODOXT MOLARS OF ^r.VMMALS. 



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

 postero-external, and the postero-internal. 



According to the theory of gradual complication, the molars 

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

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

 exception of the Monotremes, 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 pahcontological materials which the 

 Cretaceous and Lower Tertiai-y deposits of Argentina have yielded. 



The oldest fossil Mammalia of Argentina come from the varie- 

 gated sandstones which in Patngonia underlie the Gruaranian 

 formation with gigantic Dinosanrians. The best-known genus, 

 recently discovered, is the ProteodirhlpTiys pjre.cKvsor, the mandible 

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



Fiff. 1. 



ProfeoclhTel'phys prcBCUTRor : right mandibular ramus, outer aspect, four 

 times nat. size. — Lower Cretaceous ; Patagonia. 



and Microhioiherium. of the Upper Cretaceous and Eocene, connect 

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

 didelphi/s represents the most ancient stem of this group; it hns 

 at the same time many affinities with Paurodon of Marsh, and 

 other allied genera from the Upper Jurassic of North America. 



Eig. 2 repi'esents the sixth lower molar, right side, of Protcn- 

 didelphys — a, outer view, h, 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 uomeu- 

 clature. 



ma, niediaD-auterior (parHcouid). 

 ae, autero-external (protocouid). 

 ai. antero-interual (metacouid). 



pc, postero-exterual (hvpocouid). 



pi, postero-interr.al (entocouid). 



w^j, mediau-posterior ' (hvpoconulid). 



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

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

 becoming fused eitber witb cusp_pi or with cusp pe, I he latter occurrence being 

 tbe more frequent. 



