The Collaborative International Dictionary of English v.0.48:

Pyramidic \Pyr`a*mid"ic\, Pyramidical \Pyr`a*mid"ic*al\, a.
   Of or pertaining to a pyramid; having the form of a pyramid;
   pyramidal. " A pyramidical rock." --Goldsmith. "Gold in
   pyramidic plenty piled." --Shenstone. --
   Pyr`a*mid"ic*al*ly, adv. Pyr`a*mild"ic*al*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pyramidical
    adj 1: resembling a pyramid [syn: pyramidal, pyramidic,
           pyramidical]