Order Didelphimorphia

Opossums and allies

The order Didelphimorphia is the only representative of the infraclass Metatheria (marsupials) in North America. Opossums, as a group, are among the oldest, most primitive mammals of the New World. Some scientists call them living fossils because they have survived relatively unchanged for at least 50 million years. They are intermediate in many respects between the most primitive of all mammals, the egg-laying monotremes of Australia, and the higher placental mammals. Their primary character is the marsupium, or pouch, that develops on the abdomen of females. Marsupial young are born poorly developed (altricial) and the marsupium encloses the mammary glands and offers protection for the young as they complete the developmental process.

Family Didelphidae

Opossums

The family Didelphidae is a distinctive component of the New World fauna and the only family in the order Didelphimorphia. The family is represented by 17 genera and 87 species. Most members of the family, however, occur in the tropical and subtropical regions of Central and South America. The Virginia opossum is a unique exception -- it is the only marsupial to occur in North America north of Mexico.
All species of this family are small to medium size, and most, including the Virginia opossum, are characterized by the external pouch, or marsupium, in which they carry their young during early postnatal development. Most species are nocturnal or crepuscular, solitary, and omnivorous.

 

From The Mammals of Texas, Seventh Edition by David J. Schmidly and Robert D. Bradley, copyright © 1994, 2004, 2016.  Courtesy of the University of Texas Press.