Endoceras is an extinct genus of large, straight shelled cephalopods from the Middle and Upper Ordovician that gives its name to the Nautiliod order Endocerida. Shells grew to be as long as 3.5 meters The cross section in the mature portion is slightly wider than high, but is narrower laterally in the young. Sutures are straight and transverse. Endoceras has a large siphuncle, located close to the ventral margin, composed of concave segments, especially in the young but which may be tubular in the adult stage. Endocones are simple, subcircular in cross section, and penetrated by a narrow tube which may contain diaphragms reminiscent of the Ellesmerocerid ancestor.
The Piloceratidae are a compressed, rapidly expanding, cyrtoconic brevicones with holochoanitic ventral siphuncles and simple endocones. Most likely evolved from Clitendoceras, a narrow, slightly endogastric genus intermediate in form between straight shelled Proendoceras and the bulkier Piloceratidae. Found in shallow carbonate marine sediments of Demingian through the Cassinian age,.
Clitendoceras is a genus of cephalopods in the order Endocerida from the Lower Ordovician with an elongate shell with a slight downward, endogastric, curvature and a siphuncle that lies along the ventral margin. Common for endocerids, the chambers are short and the septa close spaced with sutures sloping forward across the back of the shell. Septal necks are short in the young, lengthening in the adult. Endocones are simple, but with the ventral side of the last formed projecting forward. The endosiphotube running down the middle is arched on top and somewhat flat on the lower side.
Mcqueenoceras is an extinct genus of early endocerid, a nautilus from the Ordovician period similar in overall form to Clitendoceras, from which it may have been derived. It lived during the later stages of the lower Ordovician. McQueenoceras, like Clitendoceras, has ventral siphuncle but the endocones are thicker on the ventral side and thinner on the dorsal. Also the sutures in McQueenoceras retreat rearward, forming lobes as they cross the venter.
Allotrioceras is a tubular fossil from the Middle Ordovician of the state of New York, collected by Rousseau H. Flower; included by him in the Endocerida and placed in a new family, the Allotrioceratidae. Allotrioceras is characterized by a lateral pair of subequal conical structures, resembling the endocones of endocerids, separated by a straight partition that extends more than half way across from either the dorsal or ventral side, as perceived, and runs along the length. What remains is thought to represent a siphuncle, which ranges from about 8 millimeters (0.3 in) to about 15 millimeters (0.6 in) in diameter. On the basis of diagnosis, inclusion of Allotrioceras and the Allotrioceratidae, for which it is the type, in the endocerida seems at best tentative.
Williamsoceras is an endocerid that Rousseau Flower (1968) added to his Allotrioceratidae on the basis of having a vertical partition within the siphuncle, known as a ventral process, with inter-connecting tubule-like structures along its margin where intercepted by endocones. Three species are named and described from the Garden City limestone of Whiterockian age near Logan and northern Utah, including the genotype Williamsoceras adnatum. Two other species come from the Juab limestone of near equivalent age in the southern Confusion Range in the Ibex area in western Utah.
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.
Troedssonellidae is a family of orthoceroid cephalopods from the Ordovician, derived from rod-bearing Baltoceratidae, that have a continuous lining within the siphuncle that resembles very thin and slender endocones. Shells are generally slender and orthoconic. The siphuncle is central or subcentral, composed of straight or slightly expanded segments. Septal necks generally short and connecting rings are thin. Thin cameral deposits are known, which along with the position of the siphuncle and thin connecting rings distinguishes them from the endocerids in which they have been included.
Campendoceras is a genus of proterocameroceratids from the Lower Ordovician of NW Australia and possibly Estonia that can be recognized by its slender, weakly endogastric, longiconic and annulate shell and large marginal siphuncle that contains endocones.
Endoceratidae is a family of large to very large straight shelled nautiloid cephalopods belonging to the order Endocerida that lived during the Middle and Late Ordovician. They include the largest known Paleozoic invertebrates, represented by Endoceras and Cameroceras.
Chazyoceras is a moderately large endocerid included in the Endoceratidae with a Nanno type apex and a ventral siphuncle with a holochoanitic wall, characteristic of the family. The siphuncle swelling at the apex is subtriangular in longitudinal profile. The endocones are of medium length.
Triendoceras is a genus of fairly large endocerid named by Flower (1958), included in the Endoceratidae by Teichert (1964), characterized by a holochoanitic ventral siphuncle in which the cross section through the endocones has an opening in the shape of an isosceles triangle with a sharp apex (corner) pointing down. Triendocers is found in the upper Lower Ordovician of Quebec and New York in North America and possibly in Ohio, and in eastern Europe. The type species is T. montrealense.
Perkinsocerasis an endocerid genus from the Middle Ordovician (Chazyan) of Champlain Valley established by Flower in 1976, which he added to his Allotrioceratidae based on certain similarities to Williamsoceras and Cacheoceras which had been added previously.
Bajkaloceras is a straight-shelled orthoceroid, and possibly a member of the Intejocerida, from the Angara River basin in central Russia, named by Balashov in 1962. Its age, as given in the Treatise on Invertebrate Paleontology is Arenigian.
Intejocerida is the name given to a group of generally straight shelled nautiloid cephalopods originally found in Lower and Middle Ordovician sediments in the Angara River basin in Russia; defined in the Treatise as an order, and combined there with the Endocerida in the Endoceratoidea.
Allocotoceras is an endocerid from the Lower Ordovician of Australia (Tasmania), included in the Endoceratidae, based on small, straight or gently curved siphuncles.
The Allotrioceratidae is a family of Middle Ordovician fossils, established by Rousseau Flower, 1955, originally including Allotrioceras and Mirabilocras, assigned inferentially to the Endocerida and known only from structures interpreted as siphuncles. Later Williamsoceras, Cacheoceras, and Perkinsoceras were added.
