Monotremes are sometimes called Prototheria, in contrast with Metatheria (marsupials) and Eutheria (placental mammals).    From Wordnik.com.  [Monotremata] Reference

Most zoologists would be surprised to hear that, in a few features, lagomorphs resemble marsupials more than they do placentals, and it’s on the basis of these features that some workers have actually suggested that lagomorphs might be close kin of marsupials (albeit not necessarily members of Marsupialia or even Metatheria).    From Wordnik.com.  [Archive 2006-05-01] Reference

Metatheria, 142.    From Wordnik.com.  [Thomas Henry Huxley; A Sketch Of His Life And Work] Reference

) rainforests, deserts, savannas, oceans • Metatheria (marsupials): underground, forests •.    From Wordnik.com.  [Recently Uploaded Slideshows] Reference