The whole of the remaining groups of mammals are included in a single subclass, known by the names Eutheria, Monodelphia, or Placentalia.[87] The one distinctive feature they have in common (from which the last-mentioned name is derived) is the presence of an allantoic placenta by means of which the fœtus is nourished within the uterus of the mother. Throughout the entire subclass, as a general rule, the urino-genital organs open quite independently of the rectum; the corpus callosum of the brain is well developed; the mandible does not show a marked inflection of its angle; and distinct epipubic bones are not attached to the anterior margin of the pubic symphysis. In those cases where there is a heterodont and diphyodont dentition the dental formula can be reduced to some modification of the one given on p. 25, there being only one known genus where four true molars occur, and even that not invariably. As in the Metatheria, the coracoid is reduced to a mere appendage of the scapula, and the acetabular cavity of the pelvis is imperforate. While the survivors of the other subclasses have probably been for a long time in a stationary condition, these have, as there is already good evidence to show throughout all the Tertiary geological age, and by inference for some time before, been multiplying in numbers and variations of form, and attaining higher stages of development and specialisation in various directions. They consequently exhibit far greater diversity of external or adaptive modification than is met with in either of the other subclasses,—some being fitted to live as exclusively in the water as fishes, and others to emulate the aerial flight of birds. To facilitate the study of the different component members of this large group, it is usual to separate them into certain divisions which are called “orders.” In the main zoologists are now of accord as to the general number and limits of these divisions among the existing forms, but the affinities and relationships of the orders to one another are far from being understood, and there are very many extinct forms already discovered which do not fit at all satisfactorily into any of the orders as commonly defined.