A roman moulding, called a Torus. It is composed of a semicircle and a fillet. The smicircle projects from the fillet an amount equal to its radius. The concave addition above the fillet and the projection below the half round shows a combination of mouldings used in the base of a Doric Column.
Charles William Weick Elementary Mechanical Drawing, Theory and Practice (New York: McGraw-Hill Book Company, 1915) 212