TURRITELLA. Lam. (_A little tower._) _Fam._ Turbinacea, Lam. Cricostomata, Bl.--_Descr._ Turrited, elongated, generally grooved spirally; spire pointed, consisting of numerous whorls; aperture rounded or angulated; inner and outer lips thin, confluent anteriorly; operculum horny.--_Obs._ The shells composing this well defined genus, are commonly called screws, a name to which the spiral grooves of most of the species seems to entitle them. Fig. 370, T. imbricata. TYMPANOSTOMA. Schum. (_Timbrel mouth._) POTAMIS, Brongn. TYPHIS. Montf. A genus composed of MUREX tubifer, Auct. and other similar species, which have the canal closed and a perforated tube between each varix on the angulated part of the whorls. Besides the fossil species originally described, there are now five species known, which are figured in part 200, of the Conchological Illustrations by the Author. Typhis tubifer, fig. 397. ULTIMUS. Montf. (_The last._) A genus composed of OVULUM gibbosum, Auct. fig. 443, and other species in which the canals are not distinctly defined, nor elongated. This fanciful name is given to the genus on account of its being described in the last page of the book. UMBILICATED. (_Umbilicatus._) Having an umbilicus, as Nautilus umbilicatus. UMBILICUS. (_A navel._) The hollow formed in spiral shells when the inner side of the volutions do not join each other, so that the axis is hollow. The umbilicus 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, which constitutes 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. UMBRELLA. (_A little shade._) _Fam._ Semiphyllidiana, Lam. Patelloidea, Bl.--_Descr._ Patelliform, sub-orbicular, 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, fig. 233. UNDATED. (_Unda_, a wave.) Waved. UNDULATED. (_Undulatus._) 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.