1S99.] 



PLEXODONT MOL.VRS OF MAMMALS. 



56; 



th? oldest known representative of the latter is Proteodidelphys, 

 we conclude that originally the anterior molars were composed of 



Fig. 16. 



HomnTieidus patagonkus ; second to sixth lower molars, superior aspect, 

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



the same elements as the posterior. These elements were already 

 almost suppressed in the Proteodidelphys of the beginning of 

 the Cretaceous, and had completelj' disappeared in the molars of 

 the Eocene Microbiotheridae, which in this respect resemble the 

 recent Didelphyidae. 



The traces ot' 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, whii^h 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 ai-e more simple but bear the traces of a vanished compli- 

 cation, are in an uninterrupted, closely arranged series with 

 the posterior molars, the idea arises, quite naturally, that the 

 obhque 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 want of space. The oblique 

 insertion, but not the comphcatiou, is still discernible in the Eocene 



37* 



