570 SK. r. AMEGHiivo OK THE [May 2, 



These chaDges were brought about during the Cretaceous and 

 the early portion ot 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. Einding 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 reduc- 

 tion 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 conse- 

 quence of an increase of space, the replacing molars increased in 

 size; this enlargement was accompanied byagi-adual complication, 

 giving to the molars a uniform appearance from one end of the 

 series to the other, just as during the Cretaceous. The complica- 

 tion of the anterior molars is therefore a reversion to a primitive 

 form. 



To sum up. As a result of the comparison of the palseontological 

 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 develop- 

 ment 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 j9e and pi, which are fused together. Tor 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, resem- 

 bling 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 mj>3 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 



