Eutheria

Eutheria is a grouping within the animal class of Mammalia, and was proposed by Huxley in 1880. It's nearly synonymous with the taxon of Placentalia, though it can be seen as slightly wider.

It contains all living mammals, with the exceptions of marsupials and the egg-laying monotremes. However, it also contains the nearest ancestors of placental mammals.
The earliest known representative is Eomaia from the Lower Cretaceous of China. Whilst undoutedly a eutherid, the hips of the animal are too narrowly built, to have allowed the birth of well-developed young. This strongly suggests that a placenta could have played no role in the development of babies.

The name itself means 'true beasts'. The closest living relatives of the eutherids are within Metatheria (also established by Huxley in 1880). This is nearly synonymous with the cohort of Marsupialia. Both taxon are united within the supercohort of Theria[?].

Page reference: McKenna MC & Bell SK, (1997), Classification of Mammals Above the Species Level. Columbia University Press.