Living didelphimorphs, the opossums, are a diverse group ofmarsupials, including only one family but over 60 species. 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 morphology is often referred to as "generalized," and they probably differ little in most respects from their Cretaceous ancestors. They have 5 upper and four lowerincisors(that is, they arepolyprotodont). Theircaninesare large. Molars aretritubercularwith well developedtalonids. Thedental formulais 5/4, 1/1, 3/3, 4/4 = 50. Their feet are notsyndactylous, and the first toe of their hind feet (hallux) is partially opposable. All toes except the hallux have claws; the hallux has a nail. The tail is prehensile and usually long and scaly. Opossums have relatively longrostrums, a smallbraincase, and often, a prominentsagittal 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. Opossums are generally solitary, not interacting with conspecifics except to reproduce.