From Academic Kids
Eutheria is a classification system (infraclass) nearly synonymous with Placentalia. Introduced by Thomas Henry Huxley in 1880, he meant for it to be more broad in definition than its precursor Placentalia. However, the noun placental is used almost universally to describe a placental mammal; placental mammals are rarely, if ever, referred to as eutheres. As a result, Placentalia remains the preferred choice for many scientists.
Eutheria is a taxon introduced by Thomas Henry Huxley in 1880, containing the placental mammals and the nearest ancestors of placental mammals (which are known only from the fossil record). The name itself means 'true beasts'. The closest living relatives of the eutheres are part of the subclass Marsupialia.
See also: Mammal for classification.
See also: Theria, Metatheriaca:Euteri