(class Mammalia, subclass Theria)
The infraclass that includes all of the placental mammals and which probably arose during the Cretaceous. The blastocyst develops an outer layer of cells which surrounds a small inner group from which the embryo develops. This is retained in the uterus, nourished by means of an allantoic placenta, and born in an advanced stage of development. In adults there is no marsupium and the pelvic bones form an os innominatum. The tympanic bone may be ring-like or may form a bulla, but the alisphenoid bone never forms part of the bulla. There is no in-turned angle of the jaw.
Subjects: Zoology and Animal Sciences.