XVII. On the geometrical forms of turbinated and discoid shells

Abstract

The surface of any turbinated or discoid shell may be imagined to be generated by the revolution about a fixed axis (the axis of the shell) of the perimeter of a geometrical figure, which, remaining always geometrically similar to itself, increases continually its dimensions. In discoid shells the generating figure retains its position upon the axis as it thus revolves, as in the Nautilus Pompilius (Plate IX. fig. 3.), and the Argonaut . In turbinated shells, including the great families of Trochi, Turbines, Murices and Strombi, it slides continually along the axis of its revolution (fig. 4.). In some great classes of shells, as the Ammonites, the Nautilus scrobiculatus , the Nautilus spirula , the Helix cornea , the Trochus perspectivus , the Nerita, the generating figure increases its distance from the axis at the same time that it increases its dimensions and revolves.