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Show 1899.] PLEXODONT1 MOLARS OF MAMMALS. 563 The hypselodont molars with open cavity at the base, of several Ungulates, e.g. the Toxodontia and Typotheria, show the greatest departure from the primitive type by the complete fusion of their elements; however, by means of their oldest predecessors, they can be traced to the same origin. The slightly worn molars of Morphippus imbricatus : fifth right lower molar of adult, superior aspect, uat. size.-Upper Cretaceous ; Patagonia. Fig. 10. 'rrLU *UL on Xotohippus toxodontoides: fifth right lower molar, superior aspect, twice nat. size.-Upper Eocene; Patagonia, a. Crown of homologous tooth of existing Equus caballus. the Cretaceous Toxodonts (Proadinotherium, Pronesodon) are completely similar to those of Morphippus; so that it becomes almost impossible to distinguish isolated molars of animals of the series terminating with the Equidae from those belonging to animals of the Toxodout line. Fig. 11. SYTUX^ Archceophilus patrius: fifth right lower molar, unworn (a) and worn (b), superior aspect, four times nat. size.-Upper Cretaceous ; Patagonia. ' The teeth of Typotheria are a little different. Fig. 11a repre- |
