Orthocerida

Systematic Paleontology
 * Class: Cephalopoda
 * Subclass: Nautiloidea
 * Order: Orthocerida
 * Families, see text

Orthocerida is a cephalopod order included in the Nautiloidea, fossil shells of which are typically longiconic, gradually expanding, straight or weakly curved, with smooth or variably ornamented exteriors. Chambers within the phragmocone are generally long, separated by simple bowl-shape septa. Siphuncles are typically narrow, more or less central, never marginal. Septal necks generally straight, connecting rings simple, cyclindrical or moderately expanded. Chambers (camerae) and siphuncle interiors are with internal organic deposits.

Derivation is from the ellesmerocerid Baltoceratidae which is sometimes included in the Orthocerida instead as the ancestral family. No progeny is known, the order is holophyletic.

Families included in the Orthocerida, determed by the general nature of the shell and siphuncle and by the presence or absence of internal deposits.
 * Orthoceratidae, also referred to as the Michelinoceratidae L Ord - U Trias
 * Troedssonellidae L-M Ord
 * Sactorthoceratidae	M Ord
 * Clinoceratidae	M-U Ord
 * Geisonoceratidae	M Ord - M Dev
 * Dawsonoceratidae	U Ord -- M Dev
 * Paraphramoceratidae M Sil
 * Sphooceratidae 	M. Sil
 * Offleyoceratidae	M Sil - M Dev
 * Brachycycloceratidae U Miss  - L Perm

References Treatise on Invertebrate Paleontology, part K.
 * Walter C. Sweet, 1964. Nautiloidea -Orthocerida.