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Show 1899.] PLEXOUONT M O L A R S O F M A M M A L S . 569 premolars seems to bear a relation to the greater or lesser retardation in the development of some teeth belonging to the same series. In a considerable number of cases the immediate cause of the simplification of certain molars is simply to be found in the accelerated or retarded development of neighbouring teeth. W h e n the molars find the place unoccupied, they preserve their form or even may become more complicated. Those teeth which at the moment of piercing the gum fiud the place anterior to them occupied, extend posteriorly, and vice versa, or they become reduced if the place is occupied on their anterior as well as on their posterior side. It is well known that in the majority of modern Placentals, as also in those belonging to the most recent geological periods, the adult dentition is composed of teeth belonging to two different series. The posterior, persistent teeth belong to the first series, of which the deciduous teeth also form part; while that anterior portion of the dentition which is represented by the premolars belongs to the second series, the posterior part of which, that corresponding to the persistent (true) molars, is not developed. The molars of the first series are accordingly not all in function at the same time, being developed in a very unequal manner; when the last persistent teeth come out, the anterior teeth of the same series have already been replaced by those of the second series. The same was not the case formerly. Ancient Mammals, e. g. the Nesodontidae, Adiantidae, Homalodontotheridae, Notohippidae, &c, had, during part of their life, all tbe deciduous teeth (the anterior part of the first series) in function at the same time as all the persistent teeth ; in other words, the complete first series was in function at the same time. In these families the deciduous molars, as well as tbe premolars, were well developed and always exhibited the same form from one end of the series to the other, so that the molars of the second series, replacing the deciduous teeth, occupied the same space and reached the same size. Later on, however, as a consequence of the accelerated development, by which the deciduous molars came to be shed before the animal was adult, whilst the persistent molars remained in function, these latter acquired a greater development and encroached on part of the space left free by the deciduous teeth. As to the premolars, finding the space between the canine and the first persistent molar greatly reduced, they were pressed together and had to assume an oblique position, the posterior lobe being turned towards the inner side. This oblique position of the teeth, together with the want of space necessary for their complete development, caused the reduction of their interior side and especially of the posterior lobe, which in many genera disappeared completely1. 1 All that has been stated with regard to the lower premolars applies equally also to the upper premolars, in which the atrophied lobe is the posterior, especially its inner portion. |
