306 YAGINULA. 



TJNICOENUS. Montf. Mokocekos, Auct. 



UNIO. (A pearl.) Fam. Nayades, Lam. Sul^mytilacea, BL— 

 Bescr. Inequilateral, equivalve, regular, free, pearly witMn, 

 covered by a smootli epidermis without ; umbones prominent, 

 generally corroded ; muscular impressions two in eacb valve, 

 lateral, distant ; the anterior composed of several small divi- 

 sions ; hinge varying in age, species, and individuals. — Obs. 

 The above description is framed so as to include aU 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 of Nay- 

 ades. They are aU represented in figures 140 to 152. Of 

 these fig. 145 to 148, are more generally considered as form- 

 ing the genus Unio. PL viii. 



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. 



UPPEE-VALVE. The free valve in attached bivalves. 



TJTEICULUS. Brown. A sub-genus of BuUidse, the shells 

 of which are thus described by Mr. A. Adams, in his Mono- 

 graph of BuUidse in Sowerby's Thesaurus — " Shell small, ob- 

 long-ovate ; outer lips nearly the whole length of the last 

 whorl, and entire ; spire very short, volutions prominent." 



UVIGEENA, D'Orb. A genus of microscopic Eoraminifera. 



VAGINA. Megerle. Solen vagina, Auct. 



VAGINULA. (A little sheath, the husk of corn.) Class, Ptero- 

 poda, Lam. — Descr. Pyramidal, shghtly inflated in the centre, 

 thin, fragile ; aperture oblong, with the edges turned slightly 

 outwards.— 05*. The little shells of this genus, which are only 

 known in a fossil state, differ from Cuvieria in being pointed at 

 the extremity. Pound in the tertiary beds of Bordeaux. V. 

 Daudinii, PI. xii. fig. 225. 



