The complete confirmation of Wood's (1939) statement that the "sciuromorph" zygomasseteric structure had been developed by this time in the heteromyid rodents as it had been in the early Eomyids is demonstrated in this specimen. Further, it is to be noted that the infraorbital canal is not sciuridlike, but has been forced forward on the rostrum, as in the Geomyoidea. In some ways this skull shows similarities to _Florentiamys loomisi_ Wood, of the early Miocene, which aid in determining the relationship of that unusual genus to _Heliscomys_ and to the heteromyids in general. When Wood described _Florentiamys_ the peculiar combination of characters found in this animal prompted him to speculate that: (1) It was a typical heteromyid which had secondarily developed cingula; (2) its cheek teeth were nearer the primitive pattern than were those of any other known fossil heteromyid, and that _Heliscomys_ represented a simplification in the reduction of the cingula; or (3) it was not a heteromyid, but a parallel development from the "Paramys" stock. Wood favored the second possibility. Now that a part of the skull of one species of _Heliscomys_ is known, the undivided internal cingulum that is confluent with the hypocone, the lateral compression of the deep rostrum, and the general similarity to the heteromyids appear as points in common between the two skulls, and demonstrate the closeness of _Florentiamys_ to the heteromyids. However, the specimen does not contribute anything new to use in choosing between Wood's first two postulates. In the writer's opinion the undivided internal cingulum is a primitive condition that has survived in _Florentiamys_ and _Heliscomys tenuiceps_. This common character together with the laterally compressed rostrum leads me to think that structurally, _H. tenuiceps_ is a link between _Florentiamys_ and the ancestral form of _Heliscomys_. Admittedly P4 of _Florentiamys_ seems far from the _Heliscomys_ pattern, but I think that this highly specialized structure could have been derived from _Heliscomys_ or a common ancestor. LITERATURE CITED MCGREW, PAUL O. 1941. Heteromyids from the Miocene and Lower Oligocene. Geol. Ser. of Field Mus. Nat. Hist., vol. 8, pp. 55-57, 1 fig. WOOD, ALBERT E. 1933. A New Heteromyid Rodent from the Oligocene of Montana. Jour. Mamm., vol. 14, pp. 134-141, 5 figs. 1935. Evolution and Relationship of the Heteromyid Rodents with New Forms from the Tertiary of Western North America. Annals of the Carnegie Mus., vol. 24, pp. 73-262, 157 figs. 1937. Part II. Rodentia, in The Mammalian Fauna of the White River Oligocene; by William Berryman Scott and Glenn Lowell Jepsen. Trans. Amer. Phil. Soc., n.s., vol. 28, pp. 155-269, figs. 8-70, pls. 23-33. 1939. Additional Specimens of the Heteromyid Rodent Heliscomys from the Oligocene of Nebraska. Amer. Jour. Sci., vol. 237, pp. 550-561, 11 figs. _Transmitted March 1, 1948._ 22-3342