Vertebrata Cuvier, 1812 - vertebrates
   Class Agnatha Cope, 1889 [paraphyletic]
     Subclass Cephalaspidomorpha see Class Agnatha Cope, 1889
       Order Petromyzontiformes Berg, 1940 - lampreys
         1 Family Petromyzontidae Bonaparte, 1832, sedis mutabilis - lampreys
         2 Family †Mayomyzontidae Bardack & Zangerl, 1971