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.