Source : Webster's Revised Unabridged Dictionary (1913)

Implacentalia \Im`pla*cen*ta"li*a\, n. pl. [NL. See {In-} not,
   and {Placental}.] (Zo["o]l.)
   A primary division of the Mammalia, including the monotremes
   and marsupials, in which no placenta is formed.