Small marsupials very similar to the present day South American opossums are common in Oligocene deposits. Locally they may be very numerous. When the extremely small size is considered, their relative abundance in Oligocene collections is surprising. Most of the specimens, however, are lower jaws, usually fragmentary. There are some upper dentitions and a few skulls. One skeleton has been described from the Florissant beds. The small size and fragmentary nature of the specimens make classification extremely difficult. The current view, as reflected by the taxonomy is that the North American didelphids were an extremely conservative group, hardly differing generically from the Eocene to the Miocene and with virtually no speciation in the Oligocene. McGrew in his description of Nanodelphys suggested that this idea, based on the conservatism of the dentition might be illusory. There is a rather wide diversity of habit among the living South American genera which is reflected in the foot structure rather than the dentition. A study of lower Oligocene specimens referred to Peratherium indicates rather clearly that McGrew's view is correct and that there were at least 3 genera of didelphids in the Oligocene and probably 6 or more species. It is still impossible to do more than guess at the diversity of habit because the foot structure is still unknown in most genera. Differences in the skull and upper dentition and the mandible, however, parallel those of certain of the South American genera and would seem to indicate that there was a radiation of North American opossums in the Oligocene similar in kind and extent to that which has occurred in Recent times in South America.