Richard Lydekker, 1911 Encyclopædia Britannica, Volume 18… (1911)
“ With the Monotremata (q.v.) this group has no near affinity; and while more nearly related to the Marsupialia (q.v.) , in which an imperfect allantoic placenta is sometimes developed, it is broadly distinguished therefrom by the invariable presence of a functional placenta by the aid of which the foetus is nourished throughout the greater portion of intra-uterine life. Other distinctive features by which marsupials are separated from monodelphians or placentals will be found in the article last mentioned. ”
