UNIVALVE. 



285 



UNDULATED. {UnJulatus.) Minutely waved. 



UNGUICULATED. {Unguis, a nail or hoof.) An unguiculated 

 operculum is one in which the layers are disposed laterally, and 

 the nucleus constitutes part of the outer edge. 



UNGUL1NA. Daud. (TJngula, a nail or claw.) Fam. Mactracea, 

 Lam. Conchacea, Bl. — Bescr. Equivalve, suh-orbicular, sub-equi- 

 lateral, 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 La- 

 marckian 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 gene- 

 rally 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. 



