The original diagnosis of Amphicytherura was rewritten for simplicity so that the diagnostic features would be more identifiable. Some of the original features were removed from the diagnosis (number and shape ofmarginal pore canals),while others were incorporated (central muscle scar pattern). The separation of Amphicytherura from Eucytherura is discussed by Butler and Jones (1957) and Howe and Laurencich (1958), and based both in general size (Amphicytherura is greater than Eucytherura) and hinge type (amphidont against merodont). Schizocythere Triebel and Amphicytherura present similar morphologies, but overall can be distinguished as discussed by Hanai (1970). The genus differs from Cythere by the absence of dorsal and central ridges and eye tubercle, more sparse arrangement of the central muscle scars and merodont/entomodont hinge of the latter, according to the review by Tsukagoshi and Ikeya (1987). Compared to Amphicytherura, Cythereis is distinguishable by its ornamentation (irregular, reticular or spiny,with a distinct central tubercle andwithout central ridge) and shape and arrangement of the central and frontal muscle scars, as described by Sylvester-Bradley (1948). Following the emended diagnoses by Benson and Kaesler (1963) and Ornellas and Fallavena (1978), Cytherura presents different ornamentation patterns (multi-ribbed surface) and frontal muscle scar shapes from Amphicytherura. Aracajuia and Amphicytherura were treated as separate genera by previous works (Musacchio and Simeoni, 1995, 1996, 2008). Other publications, such as those of Viviers et al. (2000) and Andreu et al. (2008), have considered Aracajuia to be a junior synonym of Amphicytherura. However, the two genera can be separated mainly by the shape of their central and ventral ridges, which are much larger in Amphicytherura than in Aracajuia. In Amphicytherura, the central ridge extends from the postero-central region to the antero-central region; in Aracajuia, the central ridge runs from the posterior to the anterior end, where it bifurcates: the upper branch usually reaches the lower one of the dorsal ridge, while the lower branch extends to the anterior end. The ventral ridge forms a carina-like process that ismuch more visible in Amphicytherura than in Aracajuia. Additional differences include the shape of the frontal muscle scar (reniform in Amphicytherura, rounded in Aracajuia) and hingement [amphidont against schizodont, although some species of Amphicytherura are also schizodont, according to Herrig (1966)]. Based on these considerations, the species remaining in Amphicytherura are shown in Table 1. The generic assignments of some Amphicytherura species are still uncertain. Amphicytherura? bartensteini (Kaye and Barker) and Amphicytherura? arcuata Luppold do not share the diagnostic ornamentation pattern (especially the shape of the three main ridges) and general layout (sub-quadratic overall shape or presence of a caudal process) with other Amphicytherura species. Amphicytherura? lecta Stepanaytys and Amphicytherura? singhi Jain are poorly described and figured in their original publications. Amphicytherura roemeri (Bartenstein) is returned to the genus Orthonotacythere, as it also does not resemble Amphicytherura. Amphicytherura kallakkudiensis Jain is re-classified as Arculicythere kallakkudiensis, following Grekoff (1963) and Ballent and Whatley (2006). Amphicytherura aculeata (Bonnema), originally placed in Eucytherura, is renamed as Apateloschizocythere aculeata, based on Bate (1972) and Clarke (1983).
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