Fam. 5. Aplustridae.—Shell partly internal, overlaid by the posterior part of the cephalic disc, spire not prominent, epipodia reflected, tentacles auriform. Single genus, Aplustrum (Fig. [286] B; subg. Hydatina).
Fam. 6. Ringiculidae.—Shell small, solid, covering all the animal; spire somewhat prominent, aperture narrow, plicated; peristome thick, sometimes channelled, cephalic disc with a kind of posterior siphon. Genera: Ringicula; Avellana (Cretaceous).
Fam. 7. Gastropteridae.—Shell completely internal, nautiloid, small; epipodia very large, rounded, united behind; cephalic disc simple. Single genus, Gastropteron.
Fam. 8. Philinidae.—Shell completely internal, thin, slightly spiral; epipodia thick, cephalic disc large, thick, simple; stomach usually with powerful gizzard. Genera: Philine (Fig. [287] E), Colpodaspis, Colobocephalus, Chelinodura, Phanerophthalmus, Cryptophthalmus.
Fam. 9. Doridiidae.—Shell completely internal, a mere pellicle with a small spiral nucleus, mantle with two posterior lobes and a caudal filament, epipodia reflected. Single genus, Doridium.
Section II. Aplysioidea.—Shell small, usually not spiral, sometimes absent, no cephalic disc, head prominent, with two pairs of tentacles, epipodia large, more or less reflected.
Fam. Aplysiidae.—Characters those of the section. Genera: Aplysia (Fig. [287] D), shell arched, flattened, animal large (the “sea hare”); Dolabella, shell subtriangular (Fig. [286] F); Dolabrifer, shell sub-quadrangular, not spiral; Notarchus, shell microscopic, spiral; Phyllaplysia, body very depressed, oval, no shell.
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.