OCR Text |
Show 1899.] PLEXODONT MOLARS OF MAMMALS. 557 three cusps of the posterior lobe are the median-posterior, the postero-external, and the postero-internal. According to tbe 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 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 paheontologieal materials which the Cretaceous and Lower Tertiary deposits of Argentina have yielded. The oldest fossil Mammalia of Argentina come from the variegated sandstones which in Patagonia underlie the Guaranian formation with gigantic Dinosaurians. The best-known genus, recently discovered, is the Proteodidelphys prcecursor, the mandible. of which, four times enlarged, is represented iu fig. 1. Eodidelphys Fig. 1. Proteodidelphyspreseursor: right mandibular ramus, outer aspect, four times nat. size.- Lower Cretaceous ; Patagonia. and Microbiotherium. of the Upper Cretaceous and Eocene, connect the former genus with the recent Didelphyidse, so that Proteodidelphys represents the most ancient stem of this group; it has at the same time many affinities with Paurodon of IViarsh, and other allied genera from the Upper Jurassic of North America. Fig. 2 represents the sixth lower molar, right side, of Proteodidelphys- a, outer view, b, 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 tbe following letters, the names in parentheses being those of Osborn's nomenclature. ma, median-anterior (paraconid). pe, postero-external (hypoconid). ae. antero-external (protoconid). pi, postero-internal (entoconid). at, antero-interual (metaconid). mp, median-posterior 1 (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, bv becoming fused either with eusp^n or with cusp pe, the latter occurrence being the more frequent. |