\Hep*tag"o*nal\, a. [Cf. F. heptagonal.]
Having seven angles or sides.
{Heptagonal numbers} (Arith.), the numbers of the series 1,
   7, 18, 34, 55, etc., being figurate numbers formed by
   adding successively the terms of the arithmetical series
   1, 6, 11, 16, 21, etc.