Abstract.-The principal conch parameters  - whorl expansion rate, whorl overlap rate, umbilical
  width, and whorl thickness - of Early and Middle Devonian ammonoids have been extensively in-
  vestigated.  Stratophenetic analyses show long-term trends in the transformations of these char-
  acters over long periods of time, but sudden and rapid reversals can also be observed.  On the basis
  of these four quantifiable conch parameters and supplementary qualitative characters, ten am-
  monoid morphs were distinguished.  Reconstruction of the evolutionary history of these morphs
  reflects the existence of two major phylogenetic lineages, both already visible in Early Devonian
  faunas.  The agoniatitid lineage is characterized by slow character development and leads to the
  Frasnian gephuroceratids; the anarcestid lineage displays rapid morphological evolution that leads
  to the late Givetian pharciceratids as well as the Middle and Late Devonian tornoceratids.  Mor-
  phological evolution is interpreted as partly limited by geometrical and physical constraints.