There are over 260 species of carnivorans, the majority of which feed primarily on meat. They have a characteristic skull shape and dentition. The odd-toed ungulates are browsing and grazing mammals. They are usually large to very large, and have relatively simple stomachs and a large middle toe. The even-toed ungulates are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in perissodactyls. There are about 220 artiodactyl species, including many that are of great economic importance to humans. Didelphimorphia is the order of common opossums of the Western Hemisphere. Opossums probably diverged from the basic South American marsupials in the late Cretaceous or early Paleocene. They are small to medium-sized marsupials, about the size of a large house cat, with a long snout and prehensile tail. There are six extant species of shrew opossum. They are small shrew-like marsupials confined to the Andes.