unio

(Lat.=a single large pearl.] 1. Zool.: River-mu el; the type-genus of Unionidse (q. v.), with more than 400 species, from all parts of the world. Shell oval or elongated, smooth, corrugated, or spiny, becoming very solid with age; anterior teeth, 1-2 or 2-2, short, irregular; posterior teeth, 1-2, elongated, laminar. Animal with the mantle margins only united between the siphonal openings; palpi long, pointed, laterally attached. [PEARL-MUSSEL.] Unio Valdensis. (From the Wealden.) 4303 2. Palæont .: Fo il species, fifty from the Wealden onward. unio-beds, s. pl. Geol.: The name given to certain beds in the Purbeck, characterized by the occurrence of species of Unio (q. v.).