The Collaborative International Dictionary of English v.0.48:

Placental \Pla*cen"tal\, a.
   1. Of or pertaining to the placenta; having, or characterized
      by having, a placenta; as, a placental mammal.
      [1913 Webster]

   2. (Zool.) Of or pertaining to the Placentalia.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Placental \Pla*cen"tal\, n. (Zool.)
   One of the Placentalia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

placental
    adj 1: pertaining to or having or occurring by means of a
           placenta; "all mammals except monotremes and marsupials
           are placental mammals" [ant: aplacental]
    n 1: mammals having a placenta; all mammals except monotremes
         and marsupials [syn: placental, placental mammal,
         eutherian, eutherian mammal]