[1] Notaspidea, also known as the sidegill slugs, was a suborder which included both sea slugs and sea snails or false limpets, marine opisthobranch gastropod molluscs in the subclass Orthogastropoda.

However, in the newer taxonomy of Bouchet & Rocroi (2005), the families Umbraculidae and Tylodinidae belong to the superfamily Umbraculoidea Dall, 1889, part of the clade Umbraculida.

Grande et al. (2004) found Umbraculoidea to be a sister clade to the Cephalaspidea (Acteonoidea excluded).

[2] The families Tylodinidae and Umbraculidae have large limpet-like external shells and a small mantle, while the species in the family Pleurobranchidae have a prominent mantle and an internal shell that becomes reduced or is lost in adults.

[3] Many species produce mantle secretions as a chemical defense against predators [4]