Menstruation
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Placentalia
Placentalia ("Placentals") is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The Placentals are partly distinguishable from other mammals in that the fetus is carried in the uterus of its mother to a relatively late stage of development. It is somewhat of a misnomer since marsupials also nourish their fetuses via a placenta.