Oxynooidea

Oxynooidea form a superfamily of small and medium-sized nudibranches, with external shell, within the superorder Sacoglossa. There are four families of shelled species: Cylindrobullidae, Volvatellidae, Oxynoidae and Juliidae.

Members of the Juliidae are also known as “bivalved gastropods” because their shells consist of two separate hinged pieces or valves. These valves are joined by a ligament, and do not look like a normal snail shell; instead the valves look almost exactly like the two hinged valves of a clam, also a bivalved mollusk. In the past the Juliidae were known only from fossil shells, but they were classified as a type of mussel, but they are not closely related, it is just a sample of convergent evolution.

The current taxonomy according to Bouchet et al. (2017) and accepted by WoRMS is:

Superfamily Oxynooidea Stoliczka, 1868 (1847)
- Family Cylindrobullidae Thiele, 1931
  - Genus Cylindrobulla P. Fischer, 1857
- Family Juliidae E. A. Smith, 1885
  - Subfamily Bertheliniinae Keen & A. G. Smith, 1961
    - Genus Berthelinia Crosse, 1875
  - Subfamily Juliinae E. A. Smith, 1885
    - Genus Julia Gould, 1862
- Family Oxynoidae Stoliczka, 1868 (1847)
  - Genus Lobiger Krohn, 1847
  - Genus Lophopleurella Zilch, 1956
  - Genus Oxynoe Rafinesque, 1814
  - Genus Roburnella Ev. Marcus, 1982
- Family Volvatellidae Pilsbry, 1895
  - Genus Ascobulla Ev. Marcus, 1972
  - Genus Volvatella Pease, 1860