1899.] 



PLEXODONT MOLAES OF MAMMALS. 



567 



the oldest known representative of the latter is ProteocliddpJiys, 

 we conclude that originally the anterior molars were conoposed of 



Pig. 16. 



Homuneulus 'patagonicus; second to sixth lower luolars, superior aspect, 

 four times nat. size. — Upper Eocene ; Patagonia. 



the same elements as the posterior. These elements were already 

 almost suppressed in the Proteodidelpliys of the beginning of 

 the Cretaceous, and had completely disappeared in the molars of 

 the Eocene Microbiotheridse, which in this respect resemble the 

 recent DidelphyidsB. 



The traces of the vanished elements are only visible on the inner 

 side, because the teeth in question are inserted obliquely, as shown 

 by the figures 1 and 15, which represent them, together with the 

 anterior root, from the outer side, the posterior one being scarcely 

 visible. On the inner side the inverse takes place, viz., the 

 posterior root occupies almost the whole of the internal face, while 

 the anterior root is almost invisible. As these anterior molars, 

 which are more simple but bear the traces of a vanished compU- 

 cation, are in an uninterrupted, closely arranged series with 

 the posterior molars, the idea arises, quite naturallj', that the 

 oblique insertion is the outcome of the want of space for 

 their development, so that the cause of the simplification of the 

 elements on the postero-internal side would be the oblique 

 insertion as a consequence of the \\ant of space. The oblique 

 iusertioij, but not the complication, is still discernible in the Eocene 



37* 



