Characteristics
Marsupials are characterized by giving birth to relatively undeveloped young. They lack a complex placenta to protect the embryo from its mother's immune system. They have a front pouch containing multiple nipples for protection and sustenance of the young.
Some common structural features can be found among marsupials. Ossified patellae are absent in most modern marsupials, though a small number of exceptions are reported. Epipubic bones are present. Marsupials (and also monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.
Read more about this topic: Marsupial