X
Xenostrobus (m.) Wilson, 1967, Proc. Malac. Soc. Lond., 37(4): , type species (OD) Volsella inconstans Dunker, 1857, (24), for 1856: 363-364, Van Diemen Island (= Tasmania); Mytilidae. Gr. xenos, h, on, foreign, strange (Liddell & Scott, 1959); Gr. strobos, o, anything twisted (Brown, 1955). 不思議な捻じれ（消化管の巻き方が他のイガイ類と逆になっている）；イガイ科
Xylophaga (f.) Turton, 1822: 253, type species (M) Teredo dorsalis Turton, 1819: 185-186. Gr.xylon, bois, wood;Gr. phago, je mange, to eat (Chenu, 1847a: 40; 1847b: 120; Woodward, 1868: 506). 木を食べる（もの）；キクイガイ科チョウチョウキクイガイ属 294
Xylophagella Meek, 1864, 7(177): 16, 34, type species (OD) Xylophaga elegantula Meek and Hayden, 1857b, 9: 141. Xylophaga Turton, 1822: 253; -ella. 小型のXylophaga；キクイガイ科
Xylotrya (f.) Menke, 1830, ed. 2: 121 [objective synonym of Pholas Linnaeus, 1758, ed. 10, 1: 669].
Xylotrya (f.) Gray, 1847, (15), for 1847: 188, type species (M) Teredo bipalmulata Lamarck, 1801: 129 [objective synonym of Bankia Gray, 1842]; Teredinidae. Gr. xylon; Gr. tryo, perforo (Herrmannsen, 1849, 2: 713). 木に穴をあける（もの）；フナクイムシ科（オオフナクイムシ属） 296
Y
Yabepecten (m.) Masuda, 1963, [n.s.], (52): 149-150, type species (OD) Pecten tokunagai Yokoyama, 1911, Jour. Geol. Soc. Tokyo, 18(208): 4, pl. 1, fig. 2 [reproduced by Yokoyama, 1920, 39(6): pl. 12, fig. 1], Pliocene Koshiba Formation; Pectinidae. Prof. Hisakatsu Yabe, Tohoku University; Pecten Müller, 1776: 248. 矢部久克教授のイタヤガイ類；イタヤガイ科ヤベホタテガイ属
Yagudinella (f.) Kafanov, 1975, Moll., their Syst., Evol. & Signif. in Nature, Zool. In-ta., Akad. NAUK SSSR: Abst. 5th Meeting, Moll. Resources: 147 (fide Vokes, 1980: 244), type species (OD) Cardium (Serripes) notabile G. B. Sowerby III, 1915, [8], 16: 169, pl. 10, fig. 9, Wakasa (Fukui Pref.), Japan; Cardiidae. Named after Saida Iliazheyinovna Yagudina (1908-1996, Russia). Yagudinaの貝；ザルガイ科（ウバトリガイ属） 174
Yamamotolepida (f.) Habe, 1976, 35(2): 38, type species (OD) Yamamotolepida semicircula Habe, 1976, 35(2): 38, pl. 1, figs. 13-14; Galeommatidae. Named after Mr. Torao Yamamoto of Seto Marine Biological Station of Kyoto University, who collected the type specimen of Y. semicirculata. 山本虎夫氏のウロコガイ類；ウロコガイ科ヤマモトウロコガイ属
Ylistrum (n.) Mynhardt & Alejandrino in Mynhardt et al., 2014, 80: 407, type species (OD) Pecten balloti Bernardi, 1861, 9: 46-48, pl. 1, fig. 1, New Caledonia; Pectinidae. Gr. verbglistro, to glide. 滑空する（もの）、GlistrumとすべきところをGr. Gをyに見立てたか；イタヤガイ科ナンヨウツキヒガイ属
Yoldia (f.) H. P. C. Möller, 1842a: 18; 1842b, 4(1): 91, type species (SD ICZN Opinion 769, 1966) Yoldia hyperborea Lovén sensu Torell, 1859, 1: 145-148 (pro Yoldia arctica Möller, 1842: 18, non Nucula arctica Gray, 1824: ccxli; Nuculanidae. Named after Alphonso D’Aguirra & Gadea Yoldi (1765-1852, Denmark). デンマークの貝類収集家Alphonso D’Aguirra & Gadea Yoldiに因む；ロウバイガイ科ナギナタソデガイ属 25
Yoldiella (f.) Verrill & Bush, 1897, 153 ([4], 3): 55-56, 63, type species (OD) Yoldia lucida Lovén, 1846, 3(6): 188-189, no. 262; Nuculanidae. Yoldia Möller, 1842a: 18; –ella. 小型のYoldia；ロウバイガイ科キビソデガイ属 26
Xenostrobus (m.) Wilson, 1967, Proc. Malac. Soc. Lond., 37(4): , type species (OD) Volsella inconstans Dunker, 1857, (24), for 1856: 363-364, Van Diemen Island (= Tasmania); Mytilidae. Gr. xenos, h, on, foreign, strange (Liddell & Scott, 1959); Gr. strobos, o, anything twisted (Brown, 1955). 不思議な捻じれ（消化管の巻き方が他のイガイ類と逆になっている）；イガイ科
Xylophaga (f.) Turton, 1822: 253, type species (M) Teredo dorsalis Turton, 1819: 185-186. Gr.xylon, bois, wood;Gr. phago, je mange, to eat (Chenu, 1847a: 40; 1847b: 120; Woodward, 1868: 506). 木を食べる（もの）；キクイガイ科チョウチョウキクイガイ属 294
Xylophagella Meek, 1864, 7(177): 16, 34, type species (OD) Xylophaga elegantula Meek and Hayden, 1857b, 9: 141. Xylophaga Turton, 1822: 253; -ella. 小型のXylophaga；キクイガイ科
Xylotrya (f.) Menke, 1830, ed. 2: 121 [objective synonym of Pholas Linnaeus, 1758, ed. 10, 1: 669].
Xylotrya (f.) Gray, 1847, (15), for 1847: 188, type species (M) Teredo bipalmulata Lamarck, 1801: 129 [objective synonym of Bankia Gray, 1842]; Teredinidae. Gr. xylon; Gr. tryo, perforo (Herrmannsen, 1849, 2: 713). 木に穴をあける（もの）；フナクイムシ科（オオフナクイムシ属） 296
Y
Yabepecten (m.) Masuda, 1963, [n.s.], (52): 149-150, type species (OD) Pecten tokunagai Yokoyama, 1911, Jour. Geol. Soc. Tokyo, 18(208): 4, pl. 1, fig. 2 [reproduced by Yokoyama, 1920, 39(6): pl. 12, fig. 1], Pliocene Koshiba Formation; Pectinidae. Prof. Hisakatsu Yabe, Tohoku University; Pecten Müller, 1776: 248. 矢部久克教授のイタヤガイ類；イタヤガイ科ヤベホタテガイ属
Yagudinella (f.) Kafanov, 1975, Moll., their Syst., Evol. & Signif. in Nature, Zool. In-ta., Akad. NAUK SSSR: Abst. 5th Meeting, Moll. Resources: 147 (fide Vokes, 1980: 244), type species (OD) Cardium (Serripes) notabile G. B. Sowerby III, 1915, [8], 16: 169, pl. 10, fig. 9, Wakasa (Fukui Pref.), Japan; Cardiidae. Named after Saida Iliazheyinovna Yagudina (1908-1996, Russia). Yagudinaの貝；ザルガイ科（ウバトリガイ属） 174
Yamamotolepida (f.) Habe, 1976, 35(2): 38, type species (OD) Yamamotolepida semicircula Habe, 1976, 35(2): 38, pl. 1, figs. 13-14; Galeommatidae. Named after Mr. Torao Yamamoto of Seto Marine Biological Station of Kyoto University, who collected the type specimen of Y. semicirculata. 山本虎夫氏のウロコガイ類；ウロコガイ科ヤマモトウロコガイ属
Ylistrum (n.) Mynhardt & Alejandrino in Mynhardt et al., 2014, 80: 407, type species (OD) Pecten balloti Bernardi, 1861, 9: 46-48, pl. 1, fig. 1, New Caledonia; Pectinidae. Gr. verbglistro, to glide. 滑空する（もの）、GlistrumとすべきところをGr. Gをyに見立てたか；イタヤガイ科ナンヨウツキヒガイ属
Yoldia (f.) H. P. C. Möller, 1842a: 18; 1842b, 4(1): 91, type species (SD ICZN Opinion 769, 1966) Yoldia hyperborea Lovén sensu Torell, 1859, 1: 145-148 (pro Yoldia arctica Möller, 1842: 18, non Nucula arctica Gray, 1824: ccxli; Nuculanidae. Named after Alphonso D’Aguirra & Gadea Yoldi (1765-1852, Denmark). デンマークの貝類収集家Alphonso D’Aguirra & Gadea Yoldiに因む；ロウバイガイ科ナギナタソデガイ属 25
Yoldiella (f.) Verrill & Bush, 1897, 153 ([4], 3): 55-56, 63, type species (OD) Yoldia lucida Lovén, 1846, 3(6): 188-189, no. 262; Nuculanidae. Yoldia Möller, 1842a: 18; –ella. 小型のYoldia；ロウバイガイ科キビソデガイ属 26