112 VARIX. 
hinge varying in age, species, and individuals. Obs. The above 
description is framed to include all the genera of Lamarckian 
Nayades, with Castatia, placed by him in his family Trigonata. 
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 under their respective 
letters. Fig. 140 to 152, represent the whole. Fig. 145 to 
148, are more generally considered as forming the genus Unio. 
UPPER VALVE. The free valve in attached bivalves. 
U'NIVALVE. (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. 
UVIGERINA. D’Orb. A genus of microscopic Foraminifera. 
VAGINA. Megerle. SoLen vagina, Auct. 
VAGIN'ULA. (A litle sheath, the husk of corn.) Class. Pteropoda, 
Lam. Descr. Pyramidal, slightly inflated in the centre, thin, 
fragile; aperture oblong, with the edges turned slightly out- 
wards. 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. 
VALVA’TA. Mull. Fam. Peristomata, Lam. Cricostomata, Bl. 
Descr. thin, turbinated; spire short, composed of three to six 
rounded whorls; aperture circular; peritreme acute, entire; 
operculum horny, spiral. Obs. This genus of small shells re- 
sembles Cyclostoma, from which the recent speciesmay be known 
by the horny texture of the external surface, being fresh-water 
shells. The fossils of course belong to the fresh-water forma- 
tions. V. piscinalis, 322. 
VALVES. (Valva, a door, or folding piece.) The two pieces 
composing a bivalve shell, which close upon each other, turning 
upon a hinge consisting of a ligament, cartilage and teeth. 
The term univalve is used to denote spiral shells. 
VALVULINA. D’Orb. A genus of microscopic Foraminifera. 
VA'RIX. (A swelling vein.) A varix is formed on the outer sur- 
face of a spiral shell by the thickened, reflected edge of a former 
aperture after fresh deposits of testaceous matter have increased 
the size by adding to the growth of the shell beyond it. In this 
manner there are frequently many varices or edges of former 
apertures in various parts of the spire and the body whorl. 
They are sometimes placed at regular distances from each 
other, as in Harpa, Fig. 419; sometimes continuous as in 
Ranella, Fig. 393; sometimes discontinuous as in Triton, Fig. 
398. 
