former taxonomy:
Class: GASTROPODA  Cuvier, 1797
Subclass: OPISTHOBRANCHIA  Milne-Edwards, 1848
Order: CEPHALASPIDEA  P. Fischer, 1883

on the recent taxonomy see WoRMS:
MolluscaBase eds. (2021). Bullina  Férussac, 1822. Accessed through:
World Register of Marine Species
at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=204861 on 2021-02-20
-