The Collaborative International Dictionary of English v.0.48:

Isomorph \I"so*morph\, n. [See Isomorphous.]
   1. A substance which is similar to another in crystalline
      form and composition.
      [1913 Webster]

   2. (Biol.) An animal, plant, or group having superficial
      similarity to another, although phylogenetically
      different.
      [Webster 1913 Suppl.]