You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
A twenty sided, 2D figure is called an icosagon.
A 2d cone is called a Light Cone. <edit: Isn't it called a circle sector?>
It is a Heptagon.
Dodecagon http://en.wikipedia.org/wiki/Dodecagon
2d
2D shapes include squares, triangles, rectangles and circles.
A ten-sided 2D polygon is known as a decagon.
A twenty sided, 2D figure is called an icosagon.
A 2D shape with 9 sides is called a Nonagon.
A rectangle is one of many names in the classification of 2D shapes as are polygons
Octagon
Square
Tetradecagon.
hexagon
A pentagon
Hexagon
Octagon