• Williamia Monterosato, 1884
• Williamia eximia (G. Nevill & H. Nevill, 1869) accepted as Akoyazara eximia (G. Nevill & H. Nevill, 1869)
• Williamia galapagana Dall, 1909 accepted as Williamia peltoides (P. P. Carpenter, 1864)
• Williamia gussoni (O. G. Costa, 1829)
• Williamia gussonii [sic] accepted as Williamia gussoni (O. G. Costa, 1829)
• Williamia gussonii var. major Pallary, 1900 accepted as Williamia gussoni (O. G. Costa, 1829)
• Williamia gussonii var. rubra Pallary, 1900 accepted as Williamia gussoni (O. G. Costa, 1829)
• Williamia japonica Habe, 1962 accepted as Williamia radiata (Pease, 1861)
• Williamia kermadecensis (Iredale, 1912) accepted as Williamia radiata (Pease, 1861)
• Williamia krebsii (Mörch, 1877)
• Williamia magellanica Dall, 1927
• Williamia maxwelli B. A. Marshall, 1981 †
• Williamia megapolitana (Wiechmann, 1868) †
• Williamia merignacensis (Cossmann & Peyrot, 1919) †
• Williamia nutata (Hedley, 1908) accepted as Williamia radiata (Pease, 1861)
• Williamia oblongata (Yokoyama, 1926) accepted as Siphonacmea oblongata (Yokoyama, 1926)
• Williamia peltoides (P. P. Carpenter, 1864)
• Williamia plioligustica Forli, Dell'Angelo, Sosso & Bonfitto, 2009 †
• Williamia polynesica Rehder, 1980 accepted as Williamia radiata (Pease, 1861)
• Williamia radiata (Pease, 1861)
• Williamia radiata Kuroda & Habe, 1961 accepted as Williamia radiata (Pease, 1861)
• Williamia radiata nutata (Hedley, 1908) accepted as Williamia radiata (Pease, 1861)
• Williamia radiata radiata (Pease, 1860) accepted as Williamia radiata (Pease, 1861)
• Williamia raincourti (Cossmann, 1885) †
• Williamia rugulosa (Koenen, 1892) † (uncertain > taxon inquirendum)
• Williamia subspiralis (P. P. Carpenter, 1864)
• Williamia taurosimplex Sacco, 1897 † (uncertain > taxon inquirendum)
• Williamia tomlini (G. C. Robson, 1913) accepted as Aporemodon tomlini G. C. Robson, 1913