Fam. 8. Lituitidae. Shell coiled in one plane with the terminal part uncoiled, aperture contracted. Lituites, Silurian. Ophidioceras, Silurian.
Fam. 9. Trochoceratidae. Shell helicoidally coiled, dextral or sinistral, the last whorl generally uncoiled. Trochoceras, Devonian. Adelphoceras, Devonian.
Fam. 10. Nautilidae. Shell coiled in one plane, aperture wide and simple, siphuncle central. Nautilus, recent. Trocholites, Silurian. Gyroceras, Silurian to Carboniferous. Hercoceras, Silurian. Ptenoceras, Devonian. Discites, Carboniferous.
Fam. 11. Bactritidae. Shell straight, conical, siphuncle narrow and marginal, necks long, infundibuliform, sutures undulating. Bactrites, Silurian and Devonian.
Sub-order 2. Ammonitoidea,—Initial chamber spheroidal; siphuncle narrow and simple; septa convex towards aperture; sutures complex.
Tribe 1. Retrosiphonata.—Siphuncular necks projecting behind the septa as in Nautiloidea. Sutures form simple undulations. Occur exclusively in Palaeozoic strata from Devonian upwards.
Fam. 1. Goniatitidae. Shell nautiloid, with simple sutures and ventral siphuncle. Goniatites, Devonian and Carboniferous. Anarcestes, Devonian.
Fam. 2. Clymeniidae. Shell nautiloid, with simple sutures, siphuncle dorsal, that is, internal. Clymenia, Upper Devonian.
Tribe 2. Prosiphonata.—Siphuncular necks projecting in front of the septa. Sutures form deeply indented lobes and saddles.
Fam. 1. Arcestidae. Globular and smooth or nearly smooth, with reduced umbilicus, terminal chamber very deep, an aptychus present. Popanoceras, Permian. Cyclolobus, Permian, Arcestes, Trias. Lobites, Trias.