A study of the species of Tornoceras, Parodiceras, and a Tornoceras and Epitornoceras from the western U.S.A. provides an independent stratigraphical zonation for the New York State Devonian. It also provides a study of allomorphosis in Tornoceras. Comments are made on the protoconch and the significance of the metamorphosis marked by the nepionic constriction in Tornoceras.

Where possible in twenty successive faunas, onto-genetic details have been elucidated, and this has been possible from protoconch upward at eleven successive stratigraphic levels. These successional ontogenies shed light on the evolution of the stock. Faunas at each level may be defined morphologically, 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 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.

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