ABSTRACT.  Ammonoid assemblages recovered from recently excavated exposures of the
 Fayetteville Shale (Chesterian, Mississippian) in its type area, northwestern Arkansas, have
 provided the first mature specimens of Fayettevillea planorbis Gordon, 1960.  Analysis of this
 material confirms the widely held view that this species with planorbid early whorls resembles
 F. friscoensis (Miller & Owen, 1944) at maturity by possession of an evolute, subdiscoidal
 conch ornamented by essentially straight, evenly spaced growth lirae, depressed whorls, and
 angular umbilical shoulders.  F. planorbis may be differentiated from F. friscoensis by deve-
 lopment in the former species of a single crenulate ridge ornamenting the umbilical shoulder
 and a mature suture with narrow, attenuate lateral lobes that are directly slightly ventrad of
 the midline of the flank.  Pseudofayettevillea gordoni n. gen & n. sp. is described from speci-
 mens in the same assemblage that also possess planorbid early whorls, but exhibit irregularly
 spaced, crenulate growth lirae through 15 mm diameter.  P. gordoni develops a subdiscoidal
 to subglobose conch with a small urnbilicus at maturity. lts umblical shoulders are ornamented
 by as many as five crenulate ridges at diameters > 10 mm, while its typically goniatitic suture
 exhibits symmetrical elements in contrast to those of mature F. planorbis.  With the rediagnosis
 of F. planorbis based on mature specimens, the Subfamily Fayettevilleinae Ruzhencev & Bogo-
 slovskaya, 1971, is transferred to the Family Glaphyritidae from the Family Rhymmoceratidae.