Most occupy Central and South America, but one species, Didelphis virginiana , occurs through most of the continental United States. Living didelphimorphs are small to medium in size. Their canines are large. Molars are tritubercular with well developed talonids . The tail is prehensile and usually long and scaly.
Opossums have relatively long rostrums , a small braincase , and often, a prominent sagittal crest .
Many opossums have a well-developed pouch, but some lack this structure. Their stomachs are simple. Most members of this group are omnivorous or carnivorous. They can be found in most neotropical habitats from sea level to over 3000m, from dry thornscrub and grassland to tropical forest. Most are at least partially arboreal, but one species has become aquatic.