The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula. In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the septa (walls) dividing the camerae (chambers). Some older studies have used the term siphon for the siphuncle, though this naming convention is uncommon in modern studies to prevent confusion with a mollusc organ of the same name.
Huronia is an actinocerid genus included in the Huroniidae along with Discoactinoceras and Huroniella,(Teichert 1964). Huronia is characterized by long siphuncle segments with the free part of the connecting rings only slightly inflated and by a narrow central canal and strongly curved radial canals located in the anterior part of each siphuncle segment
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.
The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
Lambeoceras is a genus of large actinocerids with a convexly lenticular cross section from the Upper Ordovician of North America and the sole representative of the family Lambeoceratidae.
Armenoceras is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. It is the type genus of the family Armenoceratidae.
The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial canals and a narrow space along the inner side of the connecting ring known as a paraspatium. Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm but some, like the Huroniidae of the Silurian grew significantly larger.
The Reudemannoceratidae are the ancestral and most primitive of the Discosorida, an order of cephalopods from the early Paleozoic. The Reudemannoceratidae produced generally medium-sized endogastric and almost straight shells with the siphuncle slightly ventral from the center.
Winnipegoceras is an extinct nautiloid genus from the Ordovician belonging to the Order Discosorida.
Wardoceras is an extinct nautiloid genus from the late Early Ordovician of Western Utah, assigned to the orthocerid family, Michelinoceratidae
Westonoceras is an extinct nautiloid genus from the Discosorida that lived during the Middle and Late Ordovician that has been found in North America, Greenland, and Northern Europe. It is the type genus for the Westonoceratidae
The Actinoceriatidae are a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apices and large siphuncles with widely expanded segments and a generally arcuate endosiphucular canal system. Their range is from the upper Middle Ordovician to the Lower Silurian. Actinocerids are generally straight-shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which the internal deposits are penetrated by a system of canals
The Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.
Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.
Macroloxoceras is a large pseuorthocerid from the upper Devonian of Central Colorado and Southern New Mexico with features resembling those found in actinocerids. Pseudorthocerids and actinocerids are extinct nautiloid cephalopods, generally with long straight shells and expanded siphuncle segments filled with organic deposits.
Mandaloceratidae is a family in the nautiloid cephalopod order Discosorida, from the Middle and Upper(?) Silurian characterized by short, essentially straight shells referred to as breviconic, typically with a faintly exogastric shape produced by the profile of the body chamber.
Westonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian.
Kochoceras is an extinct nautiloid genus from the later Ordovician belonging to the family Actinoceratidae and found in North America.
Carbactinoceras is a genus of Early Carboniferous (Visean) actincoceroids first found in Europe (Germany) related to Rayonnoceras but probably smaller.
Discosoridae comprise a family of endogastric discosorids, with endocones in the siphuncle, ranging from the Middle Silurian to Middle Devonian.
