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A new species of the extinct aplodontid genus Prosciurus is described from the Orellan (Oligocene) of North Dakota and Nebraska. The upper dentition of this species shares several features with the primitive Asian meniscomyine aplodontid Promeniscomys and is, therefore, believed to represent an ancestral type for later meniscomyine aplodontids.

Morphology of the four known skulls of the early Arikareean (late Oligocene) Hitonkala andersontau Macdonald indicates that this species is a member of the Florentiamyidae. Hitonkala lacks some of the derived characters of the skull present in the most-advanced florentiamyids such as Sanctimus and Florentiamys , but is more derived than the Whitneyan Kirkomys . Florentiamyids share a number of derived cranial features with the Heliscomyidae and are the immediate sister group of that family.

Additional specimens of the eomyid rodent Adjidaumo douglassi Burke from the early Arikareean (late Oligocene) Gering Formation in Nebraska imply an Arikareean (rather than Whitneyan) age for the Killdeer Formation of North Dakota, from which the holotype of this species was recovered. The new material also supports the allocation of this species to Leptodontomys, a genus previously known only from the Miocene. The Barstovian genus Pseudadjidaumo, and the North American Hemingfordian specimen referred to the otherwise European genus Eomys also are referred to Leptodontomys. These synonymies suggest a nearly continuous record for Leptodontomys from the Arikareean through the Hemphillian (late Oligocene to late Miocene). This genus belongs to the Adjidaumo-Paradjidaumo lineage of eomyids. The Eomyidae are a primitive family of generally small geomyoid rodents that range from the late-middle Eocene (Uintan) to late Miocene (Hemphillian) in North America, and as late as the Pleistocene in Europe. Their diversity was greatest in the early Oligocene (Fahlbusch, 1973, 1979), then diminished rapidly in North America, with only a few species existing through the Miocene. No lineages of eomyids previously have been traced from the Oligocene to the Miocene of North America; the majority of the Miocene species being morphologically quite distinct from the older species. The specimens discussed here provide the first record of one of the indigenous-North American Miocene genera of eomyids ex

The aplodontid, Pelycomys cf. P. placidus Galbreath, a new species of Pelycomys , and possibly a third species of this genus, are recognized from the Orellan of Nebraska. The upper dentition of Pelycomys is also decribed. The new species are morphologically distinct from previously known Orellan species from Colorado and Wyoming and represent a distinct lineage. The presence of these new species in the Great Plains indicates that aplodontids were more diverse than previously known, and that species may be geographically restricted during the middle Oligocene as has been previously shown for other families of Orellan rodents. The late Eocene rodent Spurimus is suggested as a possible direct ancestor of Pelycomys independent of Prosciurus .