Fam. 12. Proserpinidae.—Branchia replaced by a pulmonary chamber, mantle partly reflected over the shell, eyes sessile; shell depressed, discoidal, columella folded or truncated at the base, whorls with one or more internal plicae, internal partitions absorbed, no operculum. Eocene——. Single genus; Proserpina, subg. Proserpinella, Cyane, Dimorphoptychia (Eocene), and Ceres (Fig. [18]C, p. 21).
Sub-order II. Monotocardia.—Heart with one auricle, one ctenidium (the left), monopectinate, fused with the mantle (except in Valvata), one kidney, not receiving the genital products, nervous system somewhat concentrated, proboscis and penis usually present.
(a) Ptenoglossa.—Radula with formula ∞. ᴑ. ∞, teeth similar throughout, outermost largest (p. [224]).
Fam. 1. Ianthinidae.—Snout prominent, blunt, no eyes, shell helicoid, fragile, bluish, no operculum; eggs carried on a raft of vesicles attached to the foot (Fig. [42], p. 126). Pelagic only. Pliocene ——. Genera: Ianthina, Recluzia.
Fam. 2. Scalariidae.—Shell long, turriculate, whorls often partly uncoiled, with longitudinal ribs and prominent lamellae, aperture circular, operculum spiral, corneous, animal carnivorous. Ordovician ——. Principal genera: Scalaria, Eglisia, Elasmoneura (Silurian), Holopella (Silurian to Trias), Aclis.
(b) Taenioglossa.—Radula with normal formula 2.1.1.1.2, marginals sometimes multiplied (p. [223]).
Section I. Platypoda.—Foot more or less flattened ventrally.
Fam. 1. Naticidae.—Foot very large, produced before and behind, propodium reflected upon the head, eyes absent or buried in the integument, central and lateral tooth of the radula tricuspid, middle cusp strong; shell globular or auriform, outer lip simple, operculum corneous or calcareous, nucleus excentrical. Carboniferous ——. Principal genera: Natica, with many sub-genera; Ampullina (Tertiary); Amaura; Deshayesia (Tertiary); Sigaretus (Fig. [91], p. 186), shell auriform, last whorl very large, operculum much too small for the aperture.
Fam. 2. Lamellariidae.—Mantle reflected over more or less of the shell, shell delicate, no operculum. Eocene——. Principal genera: Lamellaria, shell completely internal, transparent, auriform; some species deposit their eggs on compound Ascidians (p. 74); Velutina, shell almost entirely external, paucispiral, with a thick periostracum; Marsenina, shell auriform, partly internal; Onchidiopsis, shell a membranous plate, internal.
Fam. 3. Trichotropidae.—Branchial siphon short, eyes on the outer side of the tentacles; radula closely allied to that of Velutina; shell conical, last whorl rather large, periostracum thick and hairy, operculum blunt claw-shaped, nucleus terminal. Cretaceous——. Genera: Trichotropis, Torellia.