The structural characters and phylogenetic relationships of Tertiary aplodont rodents have been discussed by several authors. Although the earliest known aplodonts occur in the John Day formation, it has been advocated, especially by Matthew,1 that the origin of the group is to be found in the family Ischyromyidae as represented by the genera Prosciurus and Paramys. Schlosser,2 however, has suggested the derivation of the John Day genus from Sciurodon of the European Phosphorites.

The new genus of rodent described in this paper is clearly ancestral to Allomys (Meniscomys) and Haplomys. Occurring in the upper Eocene stage of the Sespe deposits of southern California, it is definitely recorded as a considerably earlier type than the forms from the late Oligocene or the early Miocene. The stratigraphic position of the new aplodont in the Sespe is distinctly lower in the section than the late Eocene faunal stage now being described 3 . . .

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