''Hexameroceras'' is a genus of nautiloid

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s belonging to the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are t ...

that lived during the middle and late

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

. Its fossils have been found in North America in Indiana, New York, Ohio, and Wisconsin, and in Europe in the Czech Republic. ''Hexameroceras'' is similar to '' Tetrameroceras'' but has 3 pairs of dorsolateral sinuses (lateral embayments in the aperture) and narrow mid-dorsal projection or salient. The shell, as for the family, is breviconic, i.e. short, curved toward the apex, straight toward the aperture. The

siphuncle 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 ...

is ventral, nummuloidal and contains inwardly radial actinosiphonate deposits.

References

* Walter C. Sweet, 1964. Nautiloidea - Oncicerida, Treatise on Invertebrate Paleontoloty, Part K. Geological Society of America.
Hexameroceras
in Fossilworks Gateway. Prehistoric nautiloid genera Silurian animals Fossil taxa described in 1884 Oncocerida {{paleo-nautiloidea-stub