Theria

Theria (Greek: θηρίον, wild beast) is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians (placental mammals) and metatherians (marsupials and their ancestors). The only omitted extant mammal group is the egg-laying monotremes.

Taxonomy
The rank of 'Theria' may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000) gives the following:

In the above system Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell (1997) it is ranked as a supercohort under the subclass Theriiformes:

Another classification proposed by Luo et al. (2002) does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead.