The Collaborative International Dictionary of English v.0.48:

Echinus \E*chi"nus\, n.; pl. Echini. [L., a hedgehog, sea
   urchin, Gr. 'echi^nos.]
   1. (Zo["o]l.) A hedgehog.
      [1913 Webster]

   2. (Zo["o]l.) A genus of echinoderms, including the common
      edible sea urchin of Europe.
      [1913 Webster]

   3. (Arch.)
      (a) The rounded molding forming the bell of the capital of
          the Grecian Doric style, which is of a peculiar
          elastic curve. See Entablature.
      (b) The quarter-round molding (ovolo) of the Roman Doric
          style. See Illust. of Column
      (c) A name sometimes given to the egg and anchor or egg
          and dart molding, because that ornament is often
          identified with the Roman Doric capital. The name
          probably alludes to the shape of the shell of the sea
          urchin.
          [1913 Webster]

WordNet (r) 3.0 (2006):

echinus
    n 1: ovolo molding between the shaft and the abacus of a Doric
         column