The Collaborative International Dictionary of English v.0.48:

Heterology \Het`er*ol"o*gy\, n. [Hetero- + -logy.]
   1. (Biol.) The absence of correspondence, or relation, in
      type of structure; lack of analogy between parts, owing to
      their being composed of different elements, or of like
      elements in different proportions; variation in structure
      from the normal form; -- opposed to homology.
      [1913 Webster]

   2. (Chem.) The connection or relation of bodies which have
      partial identity of composition, but different
      characteristics and properties; the relation existing
      between derivatives of the same substance, or of the
      analogous members of different series; as, ethane, ethyl
      alcohol, acetic aldehyde, and acetic acid are in
      heterology with each other, though each in at the same
      time a member of a distinct homologous series. Cf.
      Homology.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heterology
    n 1: (biology) the lack of correspondence of apparently similar
         body parts