The earliest known terebratuloids are of late Silurian (Keyser and Downtonian) age. These and the Devonian forms are included in approximately 260 described species (11 new) which are here classified in 31 genera (8 new), 13 subfamilies (10 new), and 5 families (1 new). Morphology, stratigraphic range, geographic distribution, and evolutionary possibilities are discussed for the supra-specific categories.
In the classification the structures in the posterior of the dorsal valve were found to be of prime importance. In general, families are based on major structural differences in the cardinalia; subfamilies on features of the ventral interior and beak region, as well as differences in the cardinalia; and genera on persistent structural features in either valve and major external differences. Where the impressions of the muscles and pallial sinuses are clear and where characters of the brachidial loop are sufficiently stable they are used as supplementary criteria. For specific characters there remain minor characters in external and internal morphology and, in some genera, difference in density of punctation.
The morphology of the brachidia was determined for many genera, including Amphigenia, Rensselandia, and four of the eight new genera. The loop was found to be subject to considerable individual variation.
A key to the genera and brief discussion of preparatory techniques are included. Morphology and terminology in general are considered in glossary form, but special treatment is given delthyrial structures, external features of the cardinal margin, coloration, and shell structure. It is suggested that the Dalmanellacea may have been the ancestors of the terebratuloids.