''Pentameroceras'' is a straight to slightly exogastric breviconic

oncocerid 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 ...

from the middle

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 ...

of North America and Europe belonging to the

Trimeroceratidae Trimeroceratidae is a family of molluscs in the order Oncocerida. These molluscs were fast-moving nektobenthic carnivores. They lived in the Silurian period. Distribution Species have been found in Silurian-era strata of Canada (Québec), China ...

. This 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 ...

produced a straight to slightly exogastric shell with a variable cross section that is generally circular, in which septa are close spaced and transverse, the body chamber is inflated. The aperture, which is constricted, consists of a narrow, vertical slit-like opening extending from the ventral hyponomic sinus to a dorsal sinus and two pairs of lateral sinuses that diverge in the upper portion.

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 ...

s in the Trimeroceratidae are slender and empty.

References

* Sweet W.C. 1964. Nautiloidea-Oncocerida. Treatise on Invertebrate Paleontology, Part K. Geological Society of America and Univ. of Kansas press. Teichert & Moore, (eds)
Pentameroceras- Paleobio db
{{Taxonbar, from=Q7165021 Prehistoric nautiloid genera Oncocerida