Many ammonoid cephalopod taxa display late ontogenetic modifications
   of the shell.  These modifications include any number of the following: change
   in coiling, modification of whorl section, change in ornament, development of
   an apertural constriction, development of an apertural shell thickening, forma-
   tion of a characteristic ultimate peristome (with lappets or a rostrum, for ex-
   ample), changes in shell deposition, progressive approximation of the ultimate
   septa, progressive simplification of the ultimate few sutures, thickening of the
   last septum, und development of muscle scars.  Such modifications are related to
   the attainment of sexual maturity.  The ecologic und physiologic implications of
   these mature modifications are uncertain.
       Long ago, sexual dimorphism was suggested to account for the widespread
   existence of pairs of morphologically distinct mature ammonoids which are
   virtually identical as juveniles.  This explanation is most probably valid.
       The following genera of the family Adrianitidae display mature modifi-
   eations: Adrianites, Crimites, Epadrianites, Hoffmannia, Neocrimites, Paler-
   mites, Pseudagathiceras, Sizilites, and Texoceras.  In specimens of all of these,
   changes in coiling and an apertural constriction are present.  Changes in orna-
   ment are present in Neocrimites, Pseudagathiceras, Sizilites, und possibly Hoff-
   mannia.  Lappets are known in Adrianites, Neocrimites, and Palermites.  Changes
   in whorl cross-section occurred in Adrianites, Crimites, Hoffmannia, Palermites,
   und Sizilites.  Possible changes in shell secretion are known in Adrianites und
   Texoceras.
       Agathiceras, of the family Agathiceratidae, displays changes in coiling,
   whorl cross-section, and ornament and has an apertural construction and lappets.
   Dimorphism is present in the genus; the two forms appear to differ only in
   mature diameter.
       Three genera of the Cyclolobidae are known to bear mature modifications:
   Mexicoceras, Waagenoceras, and Cyclolobus.  All three have an apertural con-
   striction and changes in coiling and whorl cross-section.  Mexicoceras has a
   mid-ventral salient in the peristome, and Cyclolobus has lappets.
       Four genera of the Marathonitidae display mature modifications: Hyatto-
   ceras, Pseudovidrioceras, Marathonites, und Peritrochia.  All four have an
   apertural constriction.  Hyattoceras and some Marathonites display changes in
   coiling.  Hyattoccras, Pseudovidrioceras, and some Marathonites show changes
   in whorl cross-section.  Marathonites und Hyattoceras euch appear to include
   dimorphs; in both genera, the two members of a pair differ in the nature of
   the mature modifications rather than just in size.
       The genera Waagenina and Stacheoctras of the Vidricceratidae show
   mature modifications.  Both have an apertural constriction, lappets, und changes
   in whorl shape. Stacheoceras shows changes in coiling. Waagenina appears to
   include dimorphs; the two members of a pair differ in the nature of the
   mature modifications.