UNGULINA. Daud. (_Ungula_, a nail or claw.) _Fam._ Mactracea, Lam. Conchacea, Bl.--_Descr._ Equivalve, sub-orbicular, sub-equilateral, with margins entire, simple, closed all round; hinge with one short, sub-divided cardinal tooth in each valve, and a very minute additional tooth in one valve, an oblong ligamentary pit divided into two portions, one of which receives the cartilage, the external ligament is immediately below the umbones; muscular impressions, two in each valve, oblong; impression of the mantle entire. U. transversa, fig. 88. Coast of Africa. UNI-AURICULATED. Having one AURICLE. See AURICULATED. UNICORNUS. Montf. MONOCEROS, Auct. UNIO. (_A pearl._) _Fam._ Nayades, Lam. Submytilacea, Bl.--_Descr._ Inequilateral, equivalve, regular, free, pearly within, covered by a smooth epidermis without; umbones prominent, generally corroded; muscular impressions two in each valve, lateral, distant; the anterior composed of several small divisions; hinge varying in age, species, and individuals.--_Obs._ The above description is framed so as to include all the genera of the Lamarckian Nayades, together with Castalia, which are placed in the family Trigonacea, they are all fresh-water shells, commonly called fresh-water muscles. The distinctions of the various genera into which they have been divided, will be found in their respective places, and under the name Nayades. They are all represented in figures 140 to 152. Of these fig. 145 to 148, are more generally considered as forming the genus Unio. UNIOPSIS. Sw. A sub-genus of Alasmodon. Sw. p. 382. UNIVALVE. (_Unus_, one; _valva_, valve.) A shell consisting of a single piece, as distinguished from Bivalves and Multivalves, which are composed of two or more principal pieces. Spiral shells having an operculum, are called sub-bivalves by some authors. UPPER-VALVE. The free valve in attached bivalves. UVIGERINA. D'Orb. A genus of microscopic Foraminifera. VAGINA. Megerle. SOLEN _vagina_, Auct. VAGINULA. (_A little sheath, the husk of corn._) _Class_, Pteropoda, Lam.--_Descr._ Pyramidal, slightly inflated in the centre, thin, fragile; aperture oblong, with the edges turned slightly outwards.--_Obs._ The little shells of this genus, which are only known in a fossil state, differ from Cuvieria in being pointed at the extremity. Found in the tertiary beds of Bordeaux. V. Daudinii, fig. 225. VAGINULINA. D'Orb. A genus of microscopic Foraminifera.