Living mammals are divided into three groups: (1) monotremes (“prototherians”, three genera of egg-layers of the Australian realm); (2) marsupials (“metatherians”, the diverse “pouched mammals” of the Australian realm and the Americas); and (3) eutherians (“placentals”, the rest of us, with the greatest taxonomic and adaptive complexity, and having nearly cosmopolitan distribution). The marsupials plus eutherians are referred to as “therians,” a subclass that has a rich history of diversity of extinct varieties in addition to the living orders (Cassiliano and Clemens, 1979; Kraus, 1979; Kielan-Jaworowska, Eaton and Bown, 1979). The phylogenetic relationship between monotremes and modern therians is contested. Traditional wisdom had it that monotremes have been distinct from therians since virtually the beginnings of mammalian history (e.g., Marshall, 1979) in the Late Triassic or Early Jurassic. Kemp (1983), however, favored a closer relationship between monotremes and therians, with their presumed divergence at some later, but paleontologically undefined, time.