The species of Tornoceras, Parodiceras, Epitornoceras and Aulatornoceras in North America are described. The study provides an independent stratigraphical goniatite zonation, particularly for the New York State Devonian, and it also provides an analysis of allomorphis in Tornoceras. A discussion on the protoconch apparatus and the significance of the metamorphosis at the nepionic constriction in Tornoceras is given. For the Tornoceras stock descriptions are provided where possible of the ontogeny from protoconch to adult of species at eleven successive stratigraphical levels, and faunas at other levels are also described. Thus the successional ontogenies shed light on the phylogeny of the stock. Faunas at each level may be morphologically defined, but few consistently maintained evolutionary trends have been observed. Shell form seems particularly subject to independent, and probably phenotypic variation. Through the equivalents of the Middle Devonian to the lower Frasnian, protoconch width appears to increase progressively. Similarly the suture becomes more undulating, particularly with regard to the steepness of the ventrad face of the lateral lobe. Later species show reversion to early characters in these respects. The origin of Tornoceras from Parodiceras is argued, and it is considered that Tornoceras gave rise to all later members of the Tornoceratidae. A new subgenus, Linguatornoceras, is erected for Frasnian and lower Famennian tornoceratids with small lingulate lateral lobes. Seven new species and subspecies are described.