The Deciduate Mammalia

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 nearly so.

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, the Proboscidea, and the Hyracoidea.

Each of these divisions is very closely related to one of the foregoing. Thus the Carnivora approach the Cetacea; the Proboscidea, the Sirenia; and the Hyracoidea, the Ungulata.