Abstract: The Mid-Carboniferous boundary (MCB) and seven successive marker levels are
 recognizable in the ammonoid record world-wide.  The ammonoid faunas of the Lower and
 Upper Carboniferous are fundamentally different.  After the MCB, the following seven levels
 characterize the Lower Bashkirian units with the entry of: 1) Isohomoceras; 2) Homoceras; 3)
 Hudsonoceras proteus; 4) Vallites, Homoceratoides, Surenites krestovnikovi; (5)
 Phillipsoceras circumplicatile and related forms; 6) Bilinguites; and 7) Cancelloceras.  Levels
 3 to 5 are not recognizable in North America, probably due to a lacuna or unfavourable facies
 conditions associated with the Mississippian/Pennsylvanian unconformity.