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

on the recent taxonomy see WoRMS:
MolluscaBase (2018). Berthella  de Blainville, 1824. Accessed through: World Register of Marine Species
at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=138361 on 2018-04-24
-