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Show 566 SR. F. AMEGHINO ON THE [May 2, that the last deciduous molar more closely resembles the first true molar than the last premolar. Recently I wished to make sure if this fact could also be observed in Marsupials; and I am able to state that in several small species of Diclelphys the unique deciduous molar-which corresponds with the third deciduous molar of Placentals-does not at all resemble the premolar by which it is replaced, but exhibits the form of the fourth persistent tooth (true molar), which in Marsupials is homologous with the fourth deciduous molar of Placentals, i.e., belongs to the first series. These facts prove conclusively that the deciduous molars had originally the same form as the persistent (true) molars. W e next come to the question of the degree of complication of the deciduous and of the replacing molars (premolars). On looking over the whole of the Tertiary and Recent Mammals, we observe that those of the first half of Tertiary times, especially those of the Northern Hemisphere, have, generally speaking, more simple premolars than the more recent. This fact has been considered as a proof of the theory of complication; but I hold that the explanation is a very different one. Firstly, the rule is not general, there being many exceptions. Secondly, this recent complication, which is very evident in several phylogenetic lines, is but a reversion to the primitive complicated type. Of this I proceed to give proofs. The mandible of Proteodidelphys, seen from the external side (fig. 1), shows the three anterior molars of the simple form as in the Recent and Tertiary Diclelphys. However, in examining these same teeth of Proteodidelphys from the inner side, the vestiges of a complication comparable to that of the posterior molars may be seen, a complication which in this genus seems to be on its way to disappear. Fig. 15 shows the third right lower molar, seen from Fig. 15. Proteodidelphys precursor: third right lower molar, external (a) and internal (b) aspect, eight times nat. size.-Lower Cretaceous; Patagonia. the outer side (a), which is simple, and from the inner side (6), which shows the rudimentary traces of the cusps of the posterior molars ; these same rudiments are visible, although successively less accentuated, on the anterior molars, the second and the first. The molars of Didelphyidae exhibit no traces of this complication, neither are they to be seen in the Microbiotheridae of the Eocene and the Upper Cretaceous. Now, since it is evident that the Didelphyidae are the descendants of the Microbiotheridae and that |
