A b s t r a c t: Species designation of the Viséan-Namiurian genus Goniatites De Haan, 1825, has been
based dominantly on differences in conch form and shell ornament, and variations in these characteristics
have been used as stratigraphic indicators.  Suture lines have often been ignored or accorded secondary
irnportance, but they exhibit regular and predictable variations which are of biostratigraphic significance.

   An evolutionary trend in the suture lines of Gonialites from the Moorefield-Fayetteville Formations
of northern Arkansas, U.S.A., is illustrated.  The height of the median saddle of Goniatites crenistria
Phillips from the Spring Creek member of the Morefield Formation (equivalent to the P1 Zone of England)
is less than one-half the height of the ventral lobe at maturity.  In the Ruddell member of the Moorefield
Formation and in the Batesville Formation (upper P2 Zone of England), Gonialites choctawensis Shu-
mard and Gonialites granosus Portlock are characterized by sutures whose median saddles are greater
than one-half of the ventral lobe height at diameters above 20-25 mm.  Median saddle height is more than
60 percent of the lobe height in a submature Goniatites from the lower Fayetteville Formation (E1 Zone
of England).

   Validity of this trend.is suggested by Goniatites from other parts of North America, as well as by those
from the Eurasian and African continents.