Subclass †Heterostraci Lankester, 1868
   Family †Cyathaspididae Kiaer, 1932, nom. praeval.
     Subfamily †Tolypelepidinae Strand, 1934 (1932), nom. praeval. [paraphyletic]
       Genus †Ptomaspis Denison, 1963, sedis mutabilis [sister group to excluded taxa]
         †Ptomaspis canadensis Denison, 1963