Two-Field Integrable Evolutionary Systems of the Third Order and Their   Differential Substitutions
  A list of forty third-order exactly integrable two-field evolutionary systems is presented. Differential substitutions connecting various systems from the list are found. It is proved that all the systems can be obtained from only two of them. Examples of zero curvature representations with $4 \times 4$ matrices are presented. 
Anatoly G. Meshkov and Maxim Ju. Balakhnev