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Australian Marsupials
American Marsupials
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The marsupium.
The most obvious morphological feature which sets the marsupials apart from other mammalian groups is the marsupium, or pouch. However, even this diagnostic feature is not seen in all members of the clade. Some species have exposed mammary glands to which young attach to suckle, whereas in others these glands are concealed in the pouch to provide additional protection to the newborns.

Sex organ morphology.
The other main difference to placentals, in terms of morphology, is the marsupials' reproductive traits. Females have 3 vaginae and 2 uteri, but only a single opening; the cloaca. Most males have a 2 pronged penis just for the passage of sperm, and the cloaca for all waste excretion. Young are born underdeveloped (respective to placental newborns), and migrate to their mothers pouch to complete development for a number of months. More details of these processes and morphological differences are explained here.

Metabolic rates.
Metabolic rates in marsupials are also significantly lower than other mammals of similar sizes, and life-spans are often shorter.

Only the third premolars among the postcanine cheek teeth are replaced during maturation, as opposed to all cheek teeth in placentals. The metacones are equal or larger in size than the paracones on the upper molars, again different to placentals, along with the hypoconulids being closer to the entoconids than the hypoconids on the lower molars.