What is "pterotrachea"

Pterotrachea is a taxonomic genus of medium-sized to large floating sea slugs, marine pelagic gastropod molluscs in the family Pterotracheidae, which is in the infraorder Littorinimorpha. As such they are quite closely related to such families as the tritons ( Ranellidae) and the tun shells ( Tonnidae).

These pelagic slugs are not at all closely related to the pelagic opisthobranch gastropods such as the sea angels and sea butterflies.