Unio crassidens Lamarck, 1819
Lamarck, 1819. Hist. Natur. Anim. sans Vert. 6: 71, sp. 3, var. b. [in part]
Type(s): LECTOTYPE MNHN_MPL_0030, “du lac Erie” .

Unio (sec. Elliptio) crassidens — Simpson (1900), Simpson (1914)
Unio (s.s.) crassidens — Call (1900)
Unio crassidens — Ortmann (1909), Ortmann (1909), Ortmann (1909), Ortmann (1910), Vanatta (1915)
Elliptio crassidens — Ortmann (1911), Ortmann (1912), Ortmann & Walker (1922), Ortmann (1924), Ortmann (1925), ... Smith (2021), Hopper et al. (2023), Hauck et al. (2023), MolluscaBase (2024), FMCS (2025)
Elliptio niger — Ortmann (1918), Modell (1942)
Elliptio (s.s.) niger — Frierson (1927)
Elliptio (s.s.) crassidens — Thiele (1934), Haas (1969), Burch (1975)
Elliptio (s.s.) crassidens crassidens — Haas (1969), R.I. Johnson (1970)
Elliptio crassidens crassidens — R.I. Johnson (1980), Oesch (1984)

Unio (Elliptio) nigra Rafinesque, 1820
Rafinesque, 1818. Amer. Mon. Mag. 3: 355 [as ‘Potamilus niger’]. [nomen nudum]
Rafinesque, 1820. Ann. Gén. Sci. Phys. 5 (13): 291, pl. 80, figs. 1-4.
Rafinesque, 1864. [in] Binney & Tryon, reprint: 22, 37.
Type(s): LECTOTYPE ANSP_20243, River Ohio.

Unio (sec. Elliptio) crassidens — Simpson (1900), Simpson (1914)
Elliptio nigra — Utterback (1915)
Unio crassidens — Vanatta (1915)
Elliptio niger — Ortmann (1918), Modell (1942)
Elliptio crassidens — Ortmann & Walker (1922), F.C. Baker (1928), Vidrine (1993), Parmalee & Bogan (1998), Brim Box & Williams (2000), Graf & Cummings (2007), Williams et al. (2008), Watters et al. (2009), Campbell & Lydeard (2012), Williams et al. (2014), Graf & Cummings (2021), MolluscaBase (2024)
Elliptio (s.s.) niger — Frierson (1927), Haas (1930)
Elliptio (s.s.) crassidens crassidens — Haas (1969), R.I. Johnson (1970)
Elliptio (s.s.) crassidens — Burch (1975)
Elliptio crassidens crassidens — R.I. Johnson (1980), Oesch (1984)

Unio (Elliptio) nigra var. fusca Rafinesque, 1820
Rafinesque, 1820. Ann. Gén. Sci. Phys. 5 (13): 293.
Rafinesque, 1864. [in] Binney & Tryon, reprint: 39.
(intro'd as intra-sp. of Unio (Elliptio) nigra Rafinesque, 1820)

— Vanatta (1915)
Elliptio crassidens — Parmalee & Bogan (1998), Williams et al. (2008), Watters et al. (2009)

Unio (Elliptio) nigra var. maculata Rafinesque, 1820
Rafinesque, 1820. Ann. Gén. Sci. Phys. 5 (13): 293.
Rafinesque, 1864. [in] Binney & Tryon, reprint: 39.
(intro'd as intra-sp. of Unio (Elliptio) nigra Rafinesque, 1820)

— Vanatta (1915)
Elliptio (s.s.) niger — Frierson (1927)
Fusconaia subrotunda — R.I. Johnson (1980)
Elliptio crassidens — Parmalee & Bogan (1998), Williams et al. (2008), Watters et al. (2009)

Unio cuneatus Barnes, 1823
Barnes, 1823. Amer. J. Sci. 6 (1-2): 263.

Unio (sec. Elliptio) crassidens — Simpson (1900), Simpson (1914)
Elliptio nigra — Utterback (1915)
Elliptio crassidens — Ortmann & Walker (1922), F.C. Baker (1928), Parmalee & Bogan (1998), Brim Box & Williams (2000), Williams et al. (2008), Watters et al. (2009)
Elliptio (s.s.) niger — Frierson (1927), Haas (1930)
Elliptio (s.s.) crassidens crassidens — Haas (1969), R.I. Johnson (1970)
Elliptio (s.s.) crassidens — Burch (1975)
Elliptio crassidens crassidens — Oesch (1984)

Obliquaria (Aximedia) venus Rafinesque, 1831
Rafinesque, 1831. Continuation: 3.
Rafinesque, 1864. [in] Binney & Tryon, reprint: 75.

— Simpson (1900), Simpson (1914), Burch (1975)
Quadrula (Fusconia [sic]) subrotunda — Frierson (1927)
Fusconaia subrotunda — Haas (1969)
Elliptio crassidens — Parmalee & Bogan (1998), Williams et al. (2008), Watters et al. (2009)

Unio incrassatus Lea, 1840
Lea, 1840. Proc. Amer. Phil. Soc. 1: 286.
Lea, 1842. Trans. Amer. Phil. Soc. 8 [O. 3]: 217 [55], pl. 16, fig. 34.
Type(s): HOLOTYPE USNM_84537, Columbus, Ga.

Unio (sec. Elliptio) incrassatus — Simpson (1900)
Unio (sec. Elliptio) crassidens var. incrassatus — Simpson (1914)
Elliptio (s.s.) niger-incrassatus — Frierson (1927)
Elliptio crassidens incrassatus — Clench & Turner (1956)
Elliptio (s.s.) crassidens crassidens — Haas (1969), R.I. Johnson (1970)
Elliptio (s.s.) crassidens — Burch (1975)
Elliptio crassidens — Parmalee & Bogan (1998), Brim Box & Williams (2000), Williams et al. (2008), IUCN (2011), Williams et al. (2011), Williams et al. (2014)

Unio discus Sowerby, 1868
‘Lea’ Sowerby, 1868. Conch. Icon. 16: pl. 62, sp. 310; errata.
(misidentified reference to Unio discus Lea, 1838)

Unio (sec. Elliptio) crassidens — Simpson (1900), Simpson (1914)
Unio (Lampsilis) discus — Martens (1900)
Elliptio (s.s.) niger — Frierson (1927)
Elliptio (s.s.) crassidens crassidens — Haas (1969), R.I. Johnson (1970)
Elliptio (s.s.) crassidens — Burch (1975)
Elliptio crassidens — Parmalee & Bogan (1998), Watters et al. (2009)

Unio danielsii B.H. Wright, 1899
B.H. Wright, 1899. Nautilus 13(3): 31.
B.H. Wright, 1899. Nautilus 13(4): 43.
Type(s): HOLOTYPE USNM_168967, Spring Creek, Decatur Co., Georgia.

Unio (sec. Elliptio) danielsii — Simpson (1900), Simpson (1914)
Elliptio (s.s.) niger-danielsi — Frierson (1927)
Elliptio crassidens incrassatus — Clench & Turner (1956)
Elliptio (s.s.) crassidens danielsii — Haas (1969)
Elliptio (s.s.) crassidens crassidens — R.I. Johnson (1970)
Elliptio (s.s.) crassidens — Burch (1975)
Elliptio crassidens — Parmalee & Bogan (1998), Brim Box & Williams (2000), Watters et al. (2009), Williams et al. (2011), Williams et al. (2014)

Elliptio pachyodon Olson & Harbison, 1953
Pilsbry in Olson & Harbison, 1953. ANSP Monogr. 8: 447, pl. 65, fig. 8.

Elliptio (s.s.) crassidens crassidens — R.I. Johnson (1970)
Elliptio (s.s.) crassidens — Burch (1975)
Elliptio crassidens — Watters et al. (2009)
Elliptio pachyodon — MolluscaBase (2024)

Unio okefenokensis of Authors, unknown year
B.H. Wright, in USNM Coll. of Authors.
Type(s): MANUSCRIPT NAME USNM_168966, Spanish Creek, Okefenokee Swamp, Georgia.

Unio niger Conrad, 1836
‘Rafinesque’ Conrad, 1836. Monogr. Fam. Unionidae 1(6): 49, pl. 46.
‘Rafinesque’ Conrad, 1853. Proc. Acad. Nat. Sci. 6: 253.
(reference to Unio (Elliptio) nigra Rafinesque, 1820)

Unio (sec. Elliptio) crassidens — Simpson (1900)

Unio crassus Sowerby, 1868
‘Say’ Sowerby, 1868. Conch. Icon. 16: pl. 95, sp. 520.
(misidentified reference to Unio crassus Say, 1817)

Lampsilis (sec. Eurynia) ligamentinus var. gibbus — Simpson (1900)
Lampsilis (Eurynia) ligamentina var. gibba — Simpson (1914)
Lampsilis (Ortmanniana) carinata-calendis — Frierson (1927)
Elliptio (s.s.) crassidens crassidens — Haas (1969)
Elliptio crassidens — Parmalee & Bogan (1998), Watters et al. (2009)