These may be subdivided,
according to the form of the placenta, into two groups: the Zonaria
and the Discoidea.
In the former the placenta surrounds
the chorion like a hoop, leaving its ends free of villi, or
In the Discoidea
, on the other hand, the placenta takes
the form of a thick disk, which is sometimes more or less lobed.
The mammalia which possess a zonary placentation are the Carnivora,
and the Hyracoidea.
Each of these divisions is very closely related to one of the
foregoing. Thus the Carnivora
approach the Cetacea;
and the Hyracoidea,