The Collaborative International Dictionary of English v.0.48:

Metatheria \Metatheria\ prop. n.
   A subclass of pouched animals.

   Syn: subclass Metatheria.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Metatheria
    n 1: pouched animals [syn: Metatheria, subclass Metatheria]