Zool. Pl. unios (ǁ uniones). [L. ūnio a single large pearl (Pliny). Cf. UNION sb.2] A genus of freshwater bivalves typical of the family Unionidæ; a mussel belonging to this or a related genus, esp. one yielding pearls; a river-mussel, pearl-mussel.
1824. Encycl. Brit., Suppl. V. 581/1. The second [family], Uniodæ [sic], will embrace Unio,… Anodonta, and Iridina.
1834. Griffith, trans. Cuvier, XII. 387. Pearls … are more especially produced by the thick bivalve shells, such as the uniones. Ibid., 401. There is nothing to induce us to mention the Unio here, except [etc.].
1851. S. P. Woodward, Mollusca, 41. Some of the unios thicken their umbones enormously.
1899. Nature, 15 June, 151/2. The washing out of loose pearls from the unios.
b. attrib. and Comb., as Unio-fisher, mollusc, -shaped; Unio-beds (see quot. 1888).
1861. P. P. Carpenter, in Rep. Smithsonian Instit. 1860, 259. Unio-shaped shells. Ibid., 263. The musk-rats … being nature’s great Unio-fishers.
1882. Proc. Berw. Nat. Club, ix. 506. Birds … picking up the large Unio molluscs in rivers.
1888. Cassell’s Encycl. Dict., Unio-beds, … certain beds in the Purbeck, characterized by the occurrence of species of Unio.
1897. Quart. Jrnl. Geol. Soc., Index, 400/2. Unio-bed on Notowasaga River (Canada).
