Page:Natural History, Mollusca.djvu/236

224 VALVE OF CYTHERÆA. called the pallial impression, or that which marks the attachment of the mantle. On the upper side is seen the hinge, an apparatus of shelly teeth alternating with cavities, which fit, with the most accurate precision, corresponding cavities and teeth in the opposite valve. The very summit of the shell, which often, as in this instance, projects into a blunt point, bending towards one side, is termed the beak or umbo; beneath this, on the anterior side, or that towards which it turns, there is a depressed space, often very conspicuous, termed the lunule; while on the hinder side of the umbo is spread the dense elastic leathery ligament. The lunule, umbo, and ligament, constitute the dorsal or superior border, the lower side (in the figure) is the ventral or inferior border; a line drawn from one to the other of these measures the length of the shell, the width being, of course, at right angles to this, while the thickness is determined by a line from the centre of one valve to that of the opposite.