The Collaborative International Dictionary of English v.0.48:

Implacentalia \Im`pla*cen*ta"li*a\, n. pl. [NL. See In- not,
   and Placental.] (Zool.)
   A primary division of the Mammalia, including the monotremes
   and marsupials, in which no placenta is formed.
   [1913 Webster]