Tim-AURICTILATED. 305 



that the axis is hollow. The umlDilicus is marked with the letter 

 u in Helix algira, fig. 279. The term is also used to express any 

 small, neat, rounded hollow. 



UMBO. {The boss of a buckler or shield.) The point of a bivalve 

 shell above the hinge, constituting the apex or nucleus of 

 each valve, from which the longitudinal rays diverge, and the 

 lines of growth, commencing at the minutest circle, descend in 

 gradually enlarging concentric layers to the outer margin. The 

 umbones will be marked with the letter u, in Cytherea, fig. 117. 



UMBEELLA or UMBELLA. {A little shade.) Fam. Semi- 

 phyllidiana. Lam. Patelloidea, BL— Descr. Patelliform, sub-or- 

 bicular, compressed, rather irregular ; apex slightly raised, placed 

 near the centre ; margin acute ; internal surface with a central, 

 callous, coloured disc, surrounded by a continuous, irregular 

 muscular impression. — Obs. This genus is known from Patella, 

 by its continuous muscular impression. It is commonly called 

 the Chinese Umbrella shell. There are but two species at 

 present known ; the U. Mediterranea, and the U. Indica. PL 

 xii. fig. 233. 



UNDATED. (C/kt/a, a wave.) Waved. 



UNDULATED. (Undulatus.) Minutely waved. 



UNaUICULATED. {Unguis, a nail or hoof.) An unguicu- 

 lated operculum is one in which the layers are disposed late- 

 rally, and the nucleus constitutes part of the outer edge. 



UNGULINA. Daud. {Vngula, a nail or claw.) Fam. Mac- 

 tracea. Lam. Conchacea, 'Sl. — 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 Hga- 

 mentary 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. Coast of Africa. U. trans- 

 versa, PI. iv. fig. 88. 



UNl-AUEICULATED. Having one Auricle. See Aueictj- 



LATED. 



X 



