Restricted to North and South America, didelphids have radiated into a wide variety of forms. Most are omnivorous or carnivorous. Several species are arboreal; one is aquatic and has fully webbed hind feet. Didelphids can be found in most habitats from sea level to over 3000m, from dry thornscrub and grassland to tropical forest. The extinct relatives of didelphids were even more varied in their morphology and habits; one group specialized as large carnivores, with one species actually resembling sabre-toothed cats, while another group apparently converged on kangaroo rats and other desert rodents.

 

Didelphids have a full complement of teeth (five upper and four lower incisors on each side of the jaw, one canine, three premolars, and four molars). Opossums are small to medium in body size; all have five digits on fore- and hindfeet, with the first toe on the hindfoot partially opposable; all digits except the first toe on the hindfoot have claws (it has a nail). The tail is long, scaley, and prehensile in most species. Other characteristics are described under the order didelphimorphia.