Abstract: The spectacular evolutionary radiation of the chambered cephalopods
    by the early Ordovician gave rise to major stocks, the morphological characters
    of which seem related to means of overcoming the problems of poise introduced
    by buoyancy: other significant characters seem related to trim, hydrostatic
    adjustment and hysteresis or decoupling adaptations.
       The Subclass Arnmonoidea, characterized by marginal siphuncles and egg-
    shaped protoconchs achieved their primary radiation in the early Devonian but
    the functional cause of their success still has to be determined.  The major
    classification of the Ammonoidea is reviewed in the light of the major taxa in
    other groups of chambered cephalopods and their descendents.
       Morphological and stratigraphical evidence is surveyed on the origin of the
    early coiled ammonoids from straight and slightly curved bactritids in the Lower
    Devonian.  Early forms show a progression to tighter coiling until the first whorl
    is wrapped tightly around the protoconch as achieved by almost all groups by
    the Middle Devonian.  A range of modifications of shell coiling, ornament and
    sutural diversification follows.
       Evolution in the Devonian ammonoids gave rise to several quite different
    groups so that about half the suborders recognized in the Subclass as a whole
    occur in the Devonian.  This is seen as a primary exploitation of different tactical
    techniques and these are mostly shown by quite different styles of sutural
    ontogeny: these form the criteria for subordinal diagnosis.  A classification down
    to generic level is given for the ventrally siphunculate Anarcestida and Goniatitida
    and for the dorsally siphunculate Clymeniida: these are the orders recognized in
    the Devonian.  The Bactritina are included in the Anarcestida.