The large Miocene beavers (Castoridae) Anchitheriomys Roger, 1898, and Amblycastor Matthew, 1918, have been often considered as synonyms (Flynn and Jacobs 2008). Most recently, they have been distinguished as separate genera based on the morphology of the dentary: Amblycastor restricted to North America and Anchitheriormys limited to Eurasia (Mörs and Hulbert 2010). However, the dentary that was cited as evidence for the separation lacks cheek teeth and its specific identification cannot be verified. In addition, a previously undescribed specimen from the type area of “Amblycastor” fluminis Matthew, 1918, demonstrates that the morphologies used to separate these genera are not consistent with the North American specimens; thus, the two genera are synonymous and the North American species should be referred to Anchitheriomys.