Chapter XXVII: Section III: PLEAUROBRANCHOIDEA.--Dorsal region protected by a wide

_notaeum_ or dorsal covering, or by a shell; no epipodia, ctenidium large, external, between the right under surface of the notaeum or shell and the foot; head short, shell present or absent.

FAM. 1. _Pleurobranchidae._--Shell internal or absent, notaeum with spicules, radula multiseriate. Genera: _Pleurobranchus_ (Fig. 286 H), (?) _Haliotinella_, _Pleurobranchaea_, (?) _Neda_.

FAM. 2. _Runcinidae._--Branchial lamellae few, under the posterior right notaeum, no shell. Single genus, _Runcina_.

FAM. 3. _Umbrellidae._--Shell external, depressed patelliform, not covering all the animal; foot very thick, ctenidium large, head depressed, small; radula multiseriate, teeth innumerable, very small. Genera: _Umbrella_ (Fig. 5A, p. 10), _Tylodina_.