(phylum Mollusca, class Cephalopoda)
A subclass of cephalopods which generally have planispirally, tightly coiled, septate shells (although some are coiled loosely or spirally). The protoconch is globular; the shells may be either involute or evolute. Some forms have marked ventral keels; ribs and nodes may also be present. The siphuncle is variable but mainly ventral in position. Sutures are often very complex. Cameral deposits are absent. The Ammonoidea were probably tetrabranchiate cephalopods. They constitute the largest cephalopod subclass, with 163 families, including the ammonites, in which the suture lines form very complex patterns; the ceratites, in which part of the suture line is frilled; and the goniatites, which have relatively simple suture lines. They range in age from Devonian to Late Cretaceous. All members are now extinct.
Subjects: Earth Sciences and Geography — Zoology and Animal Sciences.