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 sui'e 

 it" this fact could also be observed in Marsupials ; and I am able 

 to state that in several small species of Didelphys 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 (ti'ue 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. 



We 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 Eecent 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 Eecent and Tertiary Didelphys. However, in examining these 

 same teeth of Proteodidelpliys from the inner side, the vestiges of 

 a complication comparable to that of the posteinor 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 



Proteodidelphys ■prcecursor : 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 (b), 

 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 



