There is much to be urged in favour of either view; and in adopting the former alternative, it must be borne in mind that the difference between monotremes and marsupials is vastly greater than that which separates the latter from placentals.
Further, it is considered that too much weight has been assigned to the characters distinguishing monotremes from other mammals, foetal marsupials showing a monotreme type of coracoid, while it is probable that in the long run it will be found impossible to maintain the essential dissimilarity between the milk-glands of monotremes and other mammals.
Monotremes (2 species) and marsupials (4 families and 44 species) predominate, but are not abundant.
Adult monotremes are in like case, although the duckbilled platypus (Ornithorhynchus) has teeth when young on the sides of the jaws.
Others however (inclusive of Tritylodon and Microlestes, if they be really mammals), seem nearer to the Monotremata; and the question has yet to be decided whether placentals and marsupials on the one hand, and monotremes on the other are not independently derived from reptilian ancestors.