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Show 564 SR. F. AMEGHINO ON T H E [May 2, sents the unworn fifth right lower molar of Archceophilus patrius, from the Upper Cretaceous ; the six cusps are perfectly distinguishable, although very low and disposed a little differently. The cusp mp, which is very large and completely separated from cusp pe, has moved to the internal side, and these cusps disappear without leaving any trace as soon as the teeth begin to be functional; so that the molar acquires an entirely different contour and appearance, as shown by fig. lib, representing the same tooth of an adult specimen. » In the umvorn lower molars of Prosotherium, another Cretaceous genus of tbe same order, tbe cusps ma and mp are placed towards the outer side, so that the six cusps are disposed in two longitudinal series separated by a deep longitudinal turrow. The cusps ae and ai being also higher and thicker than the others, the crown assumes a certain resemblance to that of the molar of Microlestes antiquus, a very remarkable and suggestive fact. The origin of the molars of Primates is the same. Here, too, as in the bunodont Ungulata, the mound-shaped, bulky, and thick cusps, characteristic of the omnivorous condition, are a recent and gradual acquisition. Fig. 12. -m*<r Xotopithecus fossidatus: fifth right lower molar, slightly worn, external (a) aud superior (b) aspect, four times nat. size. - Dppar Cretaceous ; Patagouia. Fig. 13. +>Z<1 vnsp. ft Pitheculus australis: fifth right lower molar, superior (a) aud external (b) aspect, four times nat. size.-Upper Eocene ; Patagonia. Fig. 12, a, b, exhibits the fifth lower molar, not much worn, of Notopithecus fossulatus, from the Upper Cretaceous, external view and upper view. This tooth shows distinctly, although not |