Fam. 3. Actinoceratidae.—Shell straight or slightly curved, siphuncle wide, contracted at the septa by obstruction-rings. Principal genera: Actinoceras, Discosorus, Huronia, Sactoceras.—Ordovician to Carboniferous.
Fam. 4. Gomphoceratidae.—Shell globular, straight or considerably curved, aperture narrowed, T-shaped, body-chamber large, siphuncle variable in position. The aperture is in some cases so narrow that probably only the arms could be protruded. Principal genus, Gomphoceras (Fig. [255]).—Silurian.
Fam. 5. Ascoceratidae.—Shell sac-like or flask-shaped, apex truncated, unknown, body-chamber occupying nearly the whole of the shell on the ventral side, contracting at the aperture, last few septa coalescing on the dorsal side and encroaching upon the body-chamber. The young form has a symmetrical shell like Orthoceras, attached to the sac-like shell above described; as growth proceeds the former portion is thrown off. Principal genera: Ascoceras, Glossoceras.—Ordovician and Silurian.
Fig. 254.—A, Section of Orthoceras, showing the septa (s, s), and siphuncle (si, si); B, portion of the exterior of Orthoceras annulatum Sowb., × ½. (Woodwardian Museum, Cambridge.)
Fam. 6. Poterioceratidae.—Shell fusiform, contracted at both ends, aperture simple, siphuncle variable in position, inflated between the septa. The form generally resembles Gomphoceras, except for the simple aperture and fusiform shape.—Ordovician to Carboniferous.
Fam. 7. Cyrtoceratidae.—Shell conical or sub-cylindrical, slightly curved, body-chamber large, siphuncle variable in position. Single genus, Cyrtoceras.—Cambrian to Carboniferous.
Fam. 8. Lituitidae.—Shell coiled in a flat, sometimes loose spiral, last whorl straight, containing the body-chamber, often greatly prolonged. Principal genera: Lituites, Ophidioceras.—Ordovician and Silurian.
Fam. 9. Trochoceratidae.—Shell helicoid, with seldom more than two whorls, dextral or sinistral, last whorl sometimes partly uncoiled. Principal genera: Trochoceras, Adelphoceras.—Ordovician to Devonian.
Fam. 10. Nautilidae.—Shell with few whorls, more or less overlapping, septa simple, siphuncle central or sub-central, aperture not contracted.