Source : Webster's Revised Unabridged Dictionary (1913)

Aplacental \Ap`la*cen"tal\, a. [Pref. a- + placental.]
   Belonging to the Aplacentata; without placenta.

Source : WordNet®

aplacental
     adj : having no placenta; "monotremes and marsupials are
           aplacental mammals" [ant: {placental}]