PLACENTAL MAMMAL

any member of the mammalian infraclass Eutheria, characterized by the presence of a placenta, which facilitates exchange of nutrients and wastes between the blood of the mother and that of the fetus. The infraclass Eutheria includes all living mammals except marsupials and monotremes. Some authorities consider the marsupials (infraclass Metatheria) to be placental mammals, but these animals have a less developed, less efficient type of placenta that limits the gestation period. The true placenta of the eutherian allows for a longer developmental period within the protection of the womb, a factor considered to have contributed to the evolutionary success of the eutherians.