| OCR Text |
Show 1899.] PLEXODONT MOLARS OF MAMMALS. 567 th? oldest known representative of the latter is Proteodidelphys, we conclude that originally the anterior molars were composed of Fig. 16. Homunculus patayonicus; 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 completely disappeared in the molars of the Eocene Microbiotheridae, which in this respect resemble the recent Didelphyidae. 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 complication, are in an uninterrupted, closely arranged series with the posterior molars, the idea arises, quite naturally, 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 want of space. The oblique insertion, but not the complication, is still discernible in the Eocene 37* |
