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Show 1899.] PLEXODONT MOLARS OF MAMMALS. 565 much accentuated, the six primitive cusps, as also a trace of the cingulum, c ; the chief difference from Proteodidelphys being seen in cusp ma, which has moved to the inner side. Fig. 13 represents the same molar of the Eocene genus Pitheculus, a Monkey of the family Homunculidae. This tooth is more square and has lost the indentation on the internal side of each lobe ; the cusps are more in the shape of mounds, whde the median anterior cusp is very small, forming part of an anterior crest, from which it is scarcely distinct. In Homunculus, of the Upper Eocene, the same tooth (fig. 14) shows the median-anterior cusp ma to have become effaced Fig. 14. Homunculus patayonicus: fifth right lower molar, superior (a) and external (b) aspect, four times nat. size.-Upper Eocene; Patagonia. by fusion with the anterior crest, whilst the tubercular or buno-dont form is more pronounced. In recent Monkeys and in M a n the transverse anterior crest, the last vestige of cusp ma, has also disappeared, there remaining only the four cusps ae, ai, pe, pi, which are in the form of mounds or tubercles almost equal in size and imparting to tbe crown the perfect omnivorous aspect. The cusp mp often remains visible, generally placed between the two posterior cusps pe and pi, but always of minute size. At different times I have supported the contention that the complicated molars of Mammalia have retained the same form from one end of the series to the other, with no other change than that of the relative size of their different parts. O n this hypothesis, the simplification of the deciduous molars and of the premolars must be considered as a secondarily acquired character, due to the want of space for the complete development of these teeth-a simplification which must have been acquired progressively from before backwards. I have insisted on the fact that the deciduous molars, although remaining in function for a short time, are almost always more complicated than those which replace them. This is in agreement with the theory of fusion and primitive complication, since the deciduous teeth are the older dentition of the two; but it is in contradiction to the theory of gradual complication. I have also drawn attention to the fact, almost universal in Placentals P R O C . Z O O L . Soc-1S99, N o . X X X V I I . 37 |