Synaptomys morgani, sp. nov. is described from the early Pleistocene (ca. 1.0–1.6 Ma) of Florida. It is more primitive in certain features than Synaptomys cooperi and Synaptomys australis, and in one evolutionary scenario may be ancestral to both, assuming S. australis is distinct from S. cooperi. The schmelzmuster, or microscopic enamel prism banding pattern, is described for samples of extinct and extant bog lemmings from North America. Two hypotheses for the origin of Synaptomys are considered. The first, a revision of the Repenning and Grady concept of a rapid origin of Synaptomys from Mictomys, holds that the Cheetah Room population of Synaptomys from Hamilton Cave, West Virginia, evolved from Mictomys landesi or a related species, probably between 1.3 and 1.6 Ma. The second proposes that Synaptomys originated from a North American species similar to Plioctomys rinkeri. Based on current information, neither hypothesis can be rejected. Mictomys kansasensis from the Java l.f. of South Dakota appears more primitive than populations of this species from other early Pleistocene Great Plains localities, as the trailing edges of triangles in upper and lower molars in the Java sample retain thicker layers of lemming enamel. The “long” condition of the lower incisor (growing end of the incisor posterior to m3), previously assumed to be primitive, appears to be derived for Synaptomys.