Question: except for​ opossums it is hard to find marsupials in the western hemisphere

Answer: Yes, Marsupials represent the clade originating from the last common ancestor of extant metatherians. Like other mammals in the Metatheria, they give birth to relatively undeveloped young that often reside in a pouch located on their mothers' abdomen for a certain amount of time. Close to 70% of the 334 extant species occur on the Australian continent (the mainland, Tasmania, New Guinea and nearby islands). The remaining 100 are found in the Americas -- primarily in South America, but thirteen in Central America, and one in North America, north of Mexico.