Specialized species like Mastodon americanus have completely lost the rudimentary premolars.
These ancient opossums have been separated generically from Didelphys (in its widest sense) on account of certain differences in the relative sizes of the lower premolars, but as nearly the whole of the species have been formed .on lower jaws, of which some hundreds have been found, it is impossible to judge how far these differences are correlated with other dental or osteological characters.
The dentition includes one pair of premolars above and below, and rooted or rootless molars with but few enamel folds.
In both genera there is only a single pair of premolars in each jaw, but in the smaller Myoscalops there are usually three pairs of these teeth.
succession does not require their replacement by premolars.