See text Penion is a genus of large marine snails, commonly known as siphon whelks, classified within the mollusc family Austrosiphonidae.

[14] Males have a long, dorso-ventrally flattened penis, and correspondingly females have a large pallial oviduct and albumen gland.

[14] However, geometric morphometric investigation of P. chathamensis indicates that secondary sexual dimorphism is not prominent for shell shape or size.

[2][11][12][13][14] Shells are fusiform with a tall spire of roughly equal height to the aperture and siphonal canal combined.

[15] During the voyage of HMS Beagle, fossils of P. subrectus were among palaeontological samples collected by Charles Darwin from the mouth of the Santa Cruz River in Argentina.

[6][9][25] Antarctoneptunea Kelletia Penion Serratifusus Aeneator Buccinulum Euthria Tasmeuthria Molecular phylogenetic trees based on mitochondrial genomic and nuclear ribosomal DNA sequence data indicate that Penion is closely related to two genera: Kelletia found in the north Pacific Ocean and Antarctoneptunea, distributed in waters surrounding New Zealand and Antarctica.,[6][9][13][25] as well as Serratifusus.

[9][14] It is possible that the earliest known fossils of Penion belonging to P. proavitus from the New Zealand Paleocene (Teurian), actually represent a stem lineage that was the common ancestor of these three genera.