Source : Webster's Revised Unabridged Dictionary (1913)

Capstone \Cap"stone`\, n. (Paleon.)
   A fossil echinus of the genus {Cannulus}; -- so called from
   its supposed resemblance to a cap.

Source : WordNet®

capstone
     n 1: a final touch; a crowning achievement; a culmination [syn: {finishing
          touch}, {copestone}]
     2: a stone that forms the top of wall or building [syn: {copestone},
         {coping stone}, {stretcher}]