Pliocene to Holocene deposits of the U.S. Atlantic Coastal Plain from Maryland to Georgia are divided into four stages and four substages using molluscan biostratigraphic data. These divisions are the Wiltonian and Burwellian Stages (early Pliocene), Gouldian and Windyan Substages of the Colerainian Stage (late Pliocene to early Pleistocene), and Myrtlean and Yongesian Substages of the Longian Stage (late Pleistocene to Holocene). These stages may be recognized from Florida to as far north as Massachusetts and will facilitate correlation with other regions.

Cheilocerataceae are unique among Pennsylvanian and Permian ammonoids in having maintained the primitive sutural condition in which the ventral lobe remained entire or, in rare cases, incipiently bifid. The mature ventral lobe of all contemporary ammonoids is either prominently bifid or weakly trifid. Identity of these younger cheilocerataceans is confirmed by an additional distinctive feature, the phyletic migration of the siphuncle from a ventral position to lie close to the dorsum within the dorsal septal flexure.

Transformation of the ancestors, the Mississippian cheiloceratacean family Prionoceratidae, to produce the root-stock of post-Mississippian Cheilocerataceae was by progenesis (paedomorphosis and neoteny). This involved linear size reduction by a factor of ten, and retention to maturity of the rounded (formerly juvenile) character of the lateral lobe.

Two families are represented in the Pennsylvanian, both probably derived from a common prionoceratid ancestor. The monotypic Maximitidae (Atokan-Missourian) are distinguished by ontogenetic migration of the siphuncle from a central to ventral-marginal position, with concurrent development of incipient bifurcation of the ventral lobe in some forms. The more diverse and longer ranging Pseudohaloritidae are represented in the Pennsylvanian (Missourian and younger) by Neoaganides, an unornamented form with simple sutures that persisted to the extinction of the superfamily in the highest Permian Changhsingian Stage. During the Permian, the Pseudohaloritidae underwent explosive diversification, displayed especially by the Wordian endemics of South China. This involved bizarre combinations of shell features that are duplicated by some Mesozoic ammonoids but are rare or unique for the Paleozoic. Included are coarse ribs and nodes, and mature modifications comprising divergent coiling, one or two subterminal constrictions, and one or two pairs of spectacular lappets. In addition both the lobes and saddles may be irregularly but extremely denticulate to give “ceratitic” and “ammonitic” sutures, and the siphuncle is generally close to the dorsum. No fully plausible explanation has yet been provided for these late evolutionary developments.

All previously described taxa in the Maximitidae and Pseudohaloritidae are reviewed. New taxa include a species of Maximites that is dimorphic, three new forms of Neoaganides from the Permian of Texas and Iran, plus the first representatives of Pseudohalorites and Shouchangoceras from North America. A new genus (Sosioceras) is established for Brancoceras pygmaeum Gemmellaro. It differs from Neoaganides in having a doubly constricted aperture at maturity and well developed runzelschicht. This monotypic genus is dimorphic also, and strong dimorphism may be characteristic of Permian Pseudohaloritidae.

Sutures provide the basis for subdivision of the Pseudohaloritidae into subfamilies. The Shouchangoceratinae are “goniatitic,” the Pseudohaloritinae are “ceratitic,” and the Lanceoloboceratinae are “ammonitic.” The inadequately known Yinoceratinae, referred previously to the Thalassoceratidae, are shown to be closely related to their geographic and stratigraphic associates, the Lanceoloboceratinae. It is doubtful whether separation of the two subfamilies can be maintained. Although features such as sculpture and degree of development of serration of the suture are highly variable at the species level, gross changes in these features and in the form of the mature aperture are nonetheless the best taxobases for genera and species since the number of lobes remained constant at eight.