Wikipedia
Pentameroceras
Pentameroceras is a straight to slightly exogastric breviconic oncocerid from the middle Silurian of North America and Europe belonging to the Trimeroceratidae.
This nautiloid cephalopod 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. Siphuncles in the Trimeroceratidae are slender and empty.