Placentalia is the name of one of the principal groups of living mammals, and is often used in scientific classification as a subclass or an infraclass. The other two extant groups are the Monotremata and the Marsupalia. There are other groups of extinct mammals. The vast majority of living mammals are placental. The Placentalia are uniquely different from all other mammals in that the fetus is nourished during gestation via the placenta. The lineage is first known from the Lower Cretaceous of China, (Eomaia).
