Fam. 4. Ceratitidae.—Shell ribbed and tuberculated, body-chamber short, lobes denticulated, saddles simple. Principal genera: Ceratites (Fig. [258], B), Trachyceras.—Principally Trias.
Fam. 5. Clydonitidae.—Shell variable in form, body-chamber short, sutural line undulated, simple. Principal genera: Clydonites, Choristoceras, Rhabdoceras, Cochloceras.—Trias.
Fam. 6. Pinacoceratidae.—Shell discoidal, usually smooth, body-chamber short, sutural line very complex, lobes numerous. Principal genera: Pinacoceras, Sageceras.—Carboniferous to Trias.
Fam. 7. Amaltheidae.—Shell broad, keeled, last whorl concealing most of the spire, sutures with auxiliary lobes, incised.—Principal genera: Amaltheus, Schloenbacia, Sphenodiscus.—Trias, Cretaceous.
Fig. 258.—Various forms of Ammonoidea: A, Goniatites crenistria J. Phil., Carb. Limestone; B, Ceratites nodosus de Hann., Muschelkalk; C, Ammonites (Parkinsonia) Parkinsoni Sowb., Inf. Oolite; D, Phylloceras helerophyllum Sowb., Upper Lias; s, s, sutural lines.
Fam. 8. Ammonitidae.—Body-chamber long, whorls narrow, uncovered, more or less ribbed, aperture simple, sutural line normal, aptychus single, corneous. Principal genera: Ammonites, Aegoceras.—Principally Lias.
Fam. 9. Lytoceratidae.—Shell discoidal, body-chamber short, aperture simple, no aptychus. Principal genera: Lytoceras, Phylloceras (Fig. [258], D).—Trias to Cretaceous.
Fam. 10. Harpoceratidae.—Shell discoidal, compressed, margin keeled, surface with straight or arched ribs, aperture with lateral projections, suture with accessory lobes, aptychus in two pieces. Principal genera: Harpoceras, Oppelia, Lissoceras.—Jurassic to Cretaceous.
