Shells thin, bulloid, with a wide aperture, and devoid of radial sculpture. Apex very posterior. « Oxynoids have a greatly reduced shell, which may be flattened. The shell covers only the visceral mass and most of the head of the retracted slug. » – Williams & Walker: “Mesoherbivore-macroalgal interactions: feeding ecology of Saccoglossan sea slugs and their effects on their food algae”, Oceanography and marine biology vol. 37, London 1999, p.91.
 

« This shell is covered with an epidermal envelope of horny consistency, as observed in Bulla hydatis; it is also extremely thin, transparent, very domed; it presents no other trace of a whorl than a protuberance directed to the left, behind and below, and terminated in a blunt point, representing the summit of other shells. The outer edge of the peristome (labrum) is sharp; the inner edge (labium) turns towards the top of the coil in a sort of siding. » – A.»D.»Krohn: “Observations sur deux nouveaux genres de Gastéropodes”, Annales des Sciences Naturelles (Zoologie) ser.3 vol.7, Paris 1847, p.53-54. The spire is narrow.
 

Shell covering only the visceral mass; « protoconch hidden in the sunken spire » (Krug & al., 2018); the spire is more rounded than in Lobiger, and the aperture windens diagonally.