• Webster's Revised Unabridged Dictionary
    • n. pl Implacentalia (Zoöl) A primary division of the Mammalia, including the monotremes and marsupials, in which no placenta is formed.
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Century Dictionary and Cyclopedia
    • implacentalia A group of mammalia constituted by the marsupials and monotremes, representing the subclasses Didelphia and Ornithodelphia, as together contrasted with the Placentalia or Monodelphia. Though the marsupials and monotremes agree with each other and differ from other mammals in some features, as the absence of a placenta, in many important respects they differ from each other as much as they do from other mammals collectively. The term Implacentalia, therefore, has no exact classificatory significance, being now only a convenient collective term for those mammals which are devoid of a placenta. Also Implacentata, Aplacentalia, Aplacentaria.
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Webster's Revised Unabridged Dictionary
NL. See In- not, and Placental