The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian, in which the connecting rings are thin and siphuncle segments are variably expanded. At present the order consists of some 16 families, a few of which, such as the Oncoceratidae, Brevicoceratidae, and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three.
Slender Oncoceratidae are those in the family Oncoceratidae, which have slender, commonly curved, shells. Some like Oocerina are gently curved, almost straight, and with only slight expansion. Others like Dunleithoceras are strongly curved with a more notable rate of expansion. Inclusion in this somewhat arbitrary category is based on illustrations in the Treatise Part K, 1964.
The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles. The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from Bassleroceras or possibly from some member of the Estonioceratidae.
Osbornoceras is a genus of Lower Silurian cyrtoconic nautiloid cephalopods known from Ohio and possibly Manitoba, one of five general currently included in the oncocerid family Karoceratidae.
Jovellania is a genus of extinct prehistoric nautiloids from the order Oncocerida known from the Lower Devonian of Europe. Nautiloids form a broad group of shelled cephalopods that were once diverse and numerous but are now represented by only a handful of species in two genera.
Ulrichoceras is recognised as the basal cyrtogomphoceratid genus, which is the source for the rest of the Cyrtogomphoceratidae as well as for the Westonoceratidae. The Cyrtogomphoceratidae are endogastric, the Westonoceratidae, exogastric.
Basslerocerida is an order of nautiloid cephalopods from the Ordovician comprising exogastric longiconic cyrtocones, that is no longer in common use.
The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoanitic marginal or subventral, tubular siphuncles.
Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian.
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
Protcycloceratidae is an extinct family of slender, commonly annulate, members of the cephalopod order Ellesmerocerida that lived during the Early Ordovician.
Oonoceras is an extinct genus of fossil cephalopods included in the nautiloid order Oncocerida and the family Oncoceratidae from the Middle Ordovician to Middle Silurian of North America and Europe, arbitrarily included in the Slender Oncoceratidae.
The Naedyceras group comprises three similar and closely related openly coiled, gyroconic, genera within oncocerid family, Brevicoceratidae: Naedyceras, Gonionaedyceras, and Gyronaedyceras.
Oxygonioceras is a genus in the Oncocerid family, Brevicoceratidae, from the Middle Silurian of North America and Europe.
The Valcouroceratidae is a family within the Oncocerida, nautiloid cephalopods from the middle and upper Ordovician, established by Rousseau Flower in 1945.
Fayettoceras is a genus in the nautiloid family Valcouroceratidae, part of the order Oncocerida, Fayettoceras has a shell which is a depressed cyrtocone with a ventral cyrtochoanitic siphuncle of elongated ovoid segments strongly contracted at the septal necks. The internal structure is unknown.
Kindleoceras is a genus of oncocerids belonging to the Valcouroceratidae, extinct nautiloid cephalopods that lived during the Middle and Late Ordovician.
Actinomorpha is a genus of valcouroceratid, order Oncocerida, from the Middle Ordovician of central North America, named by Rousseau Flower, 1943. The shell is breviconic, short and laterally compressed; the venter more narrowly rounded than the dorsum, having the effect of a modest keel in horizontal orientation. The shell is gibbous adorally, such that the body chamber is widest well behind the aperture. The siphuncle is ventral, which could also be indicative of a horizontal orientation in life, with broad, slightly inflated segments and continuous, radial, actinosiphonate, deposits.
Danoceras is an essentially straight shelled oncocerid named by Troedsson in 1926, and included in the Diestoceratidae. It has been found in the upper, and possibly middle, Ordovician of Europe, (Estonia), Greenland, and Russia,. Named species include Danoceras ravni, type, D. inutile, D. scandinavicum, and D. skalbergensis.
Charactoceras is a tarphycerid genus cephalopod which lived during the Late Ordovician.
