The cranium of the Barstovian castoroidine beaver Eucastor tortus (Leidy, 1858), and the Clarendonian species Eucastor burgensis Korth, 2002, Eucastor dividerus Stirton, 1935, Eucastor katensis Korth, 2002, and Dipoides tanneri Korth, 1998, are described and compared. The skull of E. tortus has traits that are similar to those of the Clarendonian Nothodipoides Korth, 2002, that are associated with tooth digging. Other species examined previously referred to Eucastor Leidy, 1858, are nearly identical in cranial morphology to D. tanneri and are similar to the skull of Hemphillian Dipoides Schlosser, 1902. Prodipoides, new genus, is named with Eucastor dividerus Stirton, 1935, as type species, to include the Clarendonian species as new combinations previously referred to Eucastor: Prodipoides burgensis (Korth, 2002), Prodipoides dividerus (Stirton, 1935), Prodipoides katensis (Korth, 2002), Prodipoides lecontei (Merriam, 1896), and Prodipoides phillisi (Wilson, 1968). Eucastor is limited to the nominal species E. tortus and is referred to the Tribe Nothodipoidini. Prodipoides is referred to the Tribe Castoroidini. It is evident that the tooth-digging nothodipoidines diverged from the rest of the castoroidines in the Barstovian with the occurrence of Eucastor, and that the traditional sequence of Monosaulax-Eucastor-Dipoides is unlikely.