The genus Rhectomyax was proposed by Stewart (1930) for Asaphis undulatus Gabb, 1864, a small bivalve described from fossils collected in a mine shaft sunk between BeaPs Bar and the Half Way House, not far from Texas Flat and near the Placer-Sacramento County line, California. The age of this mine shaft collection remains imprecise, but it is Late Cretaceous and probably from within the Campanian. Although Stewart's description of Rhectomyax follows his discussion of Asaphis Modeer, 1793 (Stewart, 1930, p. 285), and is not set off from other genera included by him in the “Garidae,” he ended his description with, 4T have been unable to recognize the affinities of this shell.” Rhectomyax is the oldest genus included by Keen (1969, p. N633) in the family Psammobiidae.