1899.] PLEXODONT MOLARS OP MASIilALS. 589 



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. 

 When the molars find the place unoccupied, they preserve their 

 form or even may become more compliciited. Those teeth vi'hich 

 at the moment of piercing the gum find 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 same series have already been replaced by those of the second 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 Nesodontidse, Adiantidae, Homalodoutotheridse, Notohippidse, 

 &c., had, during part of their life, all the 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 the 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 per- 

 sistent molar greatly reduced, they were pressed together and had 

 to assume an oblique position, the posterior lobe being turned 

 towards the inner side. This obUque 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 

 completely'. 



' 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. 



