Abstract.  The stratigraphical range and the boundaries of the Middle Namurian ammonoid Reticuloce-
ras Zone are reviewed, and the ammonoid occurrences indicating this interval are documented.  The rich-
est ammonoid occurrences are recovered from the South Urals, whereas the highest precision of
ammonoid-based biostratigraphy is achieved in Britain.  Three subdominant zones are recognizable within
the Reticuloceras Zone in both successions, but their boundaries do not correspond exactly.  The lower-
most occurrence of reticuloceratids (Reticuloceras ? compressum) in Western Europe occurs slightly lower
than the entry of the coarsely-ornamented and widely umbilicate ammonoid Phillipsoceras circumplicatile
which is commonly associated with the base of the Kinderscoutian.  This level apparently correlates with
the first appearance of reticuloceratids in the South Urals, but it lies above the lower boundary of the "Reti-
culoceras-Bashkortoceras Genus Zone" of RUZHENTSEV.  The appearance of  Bilinguites bilinguis marks the
beginning of the following Upper Namurian "Bilinguites-Cancelloceras Genus Zone".