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Show 570 SR. F. AMEGHINO ON THE [May 2, These changes were brought about during the Cretaceous and the early portion of the Tertiary period. In the later Tertiary a change in the opposite direction took place, viz., a progressive retardation in the evolution and the development of the persistent molars ; so that the moment arrived when all the deciduous teeth were in function, without any of the persistent teeth having made their appearance. Finding the place free, the deciduous molars were able to assume a greater development, the last of them advancing gradually backwards, thus increasing the space for the replacing molars, and diminishing in the same proportion the space destined for the persistent (true) molars. As a consequence of this reduction of space, these latter have become proportionally smaller, and in the end cut the gums successively one after the other, sometimes at rather long intervals. For the opposite reason, viz. as a consequence of an increase of space, the replacing molars increased in size; this enlargement was accompanied by a gradual complication, giving to the molars a uniform appearance from one end of the series to tbe other, just as during the Cretaceous. The complication of the anterior molars is therefore a reversion to a primitive form. To sum up. As a result of the comparison of the palaeontological materials with those furnished by recent Mammals, it can be stated that, in the same proportion as the duration in function of the deciduous molars decreases, the space assigned to the replacing molars also decreases ; and in the same proportion as the development of the persistent molars is retarded, the space occupied by the deciduous molars and the premolars is increased. This discovery explains a number of facts which have hitherto remained almost incomprehensible. I shall confine myself to a few examples which are easily understood. The third lobe of the last lower molar of Ungulates represents the median posterior cusp mp, which was enabled to assume this greater development because there are no other teeth behind to prevent it. In the other molars this cusp is, on the contrary, obliged to maintain its median position between the posterior cusps pe and pi, which are fused together. For the same reason the posterior lobe is to be seen also in the last deciduous lower molar of recent Ungulates, since in the latter this tooth remains for a long time in function, before the first persistent tooth makes its appearance. As a consequence, in these Mammals the last deciduous molar differs both from the one by which it is replaced (the fourth premolar), and from the first persistent molar, resembling the last persistent molar. In the primitive Ungulates, on the contrary, which had all the teeth of the first series in function at the same time, the last deciduous molar could not extend posteriorly, its cusp mp being prevented by the next following molar; and therefore the tooth in question (the last deciduous molar) is different from the last persistent and resembles the first persistent and the fourth replacing molar. On examining the mandible of a young sheep having the three |