preciosus Siphoniulus is a poorly known genus of millipede containing only two living species: S. alba from Indonesia, and S. neotropicus from Mexico and Guatemala.

Males possess a single pair of gonopods (modified copulatory legs) consisting of the anterior limbs on the seventh segment, and which are partially recessed into the body.

[3] Males of either species were unknown until 2003, when samples of leaf litter from Mexico sorted by researchers at the Field Museum of Natural History revealed new specimens of S. neotropicus, allowing for complete anatomical descriptions and characterization of the gonopods.

[2] Polyxenida Pentazonia Siphoniulida Chordeumatida Platydesmida Siphonophorida Polyzoniida Julida Spirobolida Spirostreptida Stemmiulida Callipodida Polydesmida Polyxenida Pentazonia Siphoniulida Stemmiulida Polydesmida Polyzoniida Siphonophorida Siphonocryptida Platydesmida Chordeumatida Callipodida Spirobolida Spirostreptida Julida Siphoniulids have been classified in various, conflicting placements within the Helminthomorpha ("worm-like" millipedes) since their initial description, and their relation to the rest of millipedes is still unresolved.

[5] The following year, the American entomologist Orator F. Cook considered Stemiulids as "suborder Siphoniuloidea", closely related to Julidans and Spirostreptidans (a grouping termed Diplochaeata).

In the first cladistic study of millipedes, Enghoff could only place Siphoniulids as incertae sedis within the Helminthomorpha, but "probably... a specialized subordinate taxon within some juliform or colobognathan order".