Source : Webster's Revised Unabridged Dictionary (1913)

   2. (Crystallog.) Same as {Tetragonal}.

   {Pyramidal numbers} (Math.), certain series of figurate
      numbers expressing the number of balls or points that may
      be arranged in the form of pyramids. Thus 1, 4, 10, 20,
      35, etc., are triangular pyramidal numbers; and 1, 5, 14,
      30, 55, etc., are square pyramidal numbers.