The Collaborative International Dictionary
Aplacental
Aplacental \Ap`la*cen"tal\, a. [Pref. a- + placental.] Belonging to the Aplacentata; without placenta. [1913 Webster] ||
Wiktionary
aplacental
a. Lacking a placenta; belonging to the Aplacentata.
WordNet
aplacental
adj. having no placenta; "monotremes and marsupials are aplacental mammals" [ant: placental]