Complete the analogy: Cube : Square :: Cylinder : ?

A cube is a solid and a square is the flat shape you find on each of its faces, so the second word names the flat shape carried by the faces of the solid. The flat faces of a cylinder are its top and its base, and both are circles. The traps: a cone and a sphere are solids, like the cube, so they answer a completely different kind of question — an analogy of this shape must move from solid to flat shape, not from solid to solid; a triangle is the flat shape on the slanting faces of a square pyramid, borrowed here from the wrong solid.