In the British Museum, Dept. Britain, Europe and Prehistory, this two-handed vase is part of Waddesdon Bequest. It was carved from a single block of honey-coloured agate and richly decorated. The vase is assumed to be of Roman origin (5th century), and has later been decorated in England (presumably 19th century). The long curatorial description details it in every way, and from it I take the part describing the location of a snail : “The disc itself is similarly decorated only on one side; the engraved design, filled with brilliant translucent enamels, has a wide border around the circumference so that when the circular fillet of gold is inserted it can be pressed against the perimeter of the disc without harming or obscuring the enamelled decoration. The latter comprises small vine branches with bunches of grapes, leaves and tendrils; three of the branches curl to form three complete circles abutting a smaller circle in the centre. Within the branches are depicted a dragon-fly, a snail and a butterfly, each alternating with a perching bird”.
The snail itself it rather stylised (which seems natural given the material and the size), with two tentacles and a sinistral shell.
 BML, inv. WB.68. http://to.ly/Jds2, http://to.ly/Jdlp.