Valve, the whole of univalve shells, of shells in one piece; and the half of bivalves, or shells in two divisions, &c. Varices, longitudinal ribs in univalve shells. Variety, is when one species differs some little degree from that of another. Vaulted, like the roof of one’s mouth. Venter, the belly, situated in the body of the shell; being the most prominent part when the aperture is turned to the observer. Ventral, belonging to the belly. Ventricose, inflated, swelled in the middle. Vermiform, worm-shaped. Vertex, in the Patella the top or most prominent part, situated in general nearly in the middle. In the genus Bulla it is used for the apex. Verrucose, warted. Verticulated, whorled. Umbilicated, having a depression in the centre like a navel. Umbo, in bivalve shells, the round part which turns over the hinge. Umbonate, bossed, having a raised knob in the centre Undulated, waved, having a waved surface. Ungulate, shaped like a horse’s hoof. Unilocular, with a single chamber or compartment. Univalve, shells consisting of one valve or piece. Volutions, the wreaths or turnings of the shells of univalves. Urceolate, swelling in the middle like a pitcher. Vulva, a spatulated mark in several bivalve shells; formed when the valves are united on the posterior and anterior slopes. W. Whorl, one of the wreaths or turnings of the spire of univalves. Z. Zigzag, having contrary turnings and windings. Zoned, surrounded with one or more girdles. INDEX TO THE CLASSES, FAMILIES, GENERA, AND PLATES. [N.B.—The Classes are printed in Small Capitals, the Families in Italics, and the Genera in the Ordinary Type.]