Where does “Placentalia” come from?

Placentalia (Translingual) comes from New Latin placentālis, from Latin placenta, from Ancient Greek πλακόεντα, from Ancient Greek πλακοῦς, from Ancient Greek πλακόεις, from Ancient Greek πλάξ, from Proto-Hellenic pləks, from Proto-Indo-European pleh₂- — flat.

Placentalia (Translingual): The clade originating with the last common ancestor of extant eutherians