I'm presuming you mean the three different subcatagories or taxa as there around 25/26 mammalian orders. The three taxa are - Prototheria or Monotreme = Mammals that lay eggs Metatheria Marsupial = Mammals with external development pouches.
primary division of the sea, which includes the whole mass of water subdivided into neritic and oceanic zones; also pertaining to the open sea (open water)
