Theria (superclass Gnathostomata, class Mammalia) A subclass that comprises the three infraclasses Pantotheria (extinct ancestral forms), Metatheria (marsupials), and Eutheria (placental mammals), distinguished from the Prototheria principally by their teeth and by the structure of the sides of the skull, formed in therians by large squamosal and alisphenoid bones.
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