Fam. 10. Littorinidae.—Proboscis short, broad, tentacles long, eyes at their outer bases, penis behind the right tentacle; reproduction oviparous or ovoviviparous, radula very long; shell turbinate, solid, columella thickened, lip simple, operculum corneous, nucleus excentrical. Jurassic——. Principal genera: Littorina (radula, Fig. [16], p. 20), Cremnoconchus (p. 16), Fossarina; Tectarius, shell tubercled or spinose; Risella, base slightly concave; Lacuna, shell thin, grooved behind the columellar lip.
Fam. 11. Fossaridae.—Shell turbinate, solid, small, white, spirally ribbed, outer lip simple. Miocene——. Principal genus, Fossarus.
Fam. 12. Cyclophoridae.—Ctenidium replaced by a pulmonary sac, tentacles long, thread-like (radula, Fig. [17], p. 21); shell variously spiral, peristome round, often reflected, operculum circular. Terrestrial only. Cretaceous——. (i.) Pomatiasinae, shell high, conical, longitudinally striated, operculum consisting of two laminae united together. Single genus, Pomatias. (ii.) Diplommatininae, shell more or less pupiform, peristome thickened or reflected, often double. Genera: Diplommatina (subg., Nicida, Palaina, Paxillus, Arinia), shell dextral or sinistral, small, columella often denticulated; Opisthostoma (Fig. [208], p. 309), last whorl disconnected, often reflected back upon the spire. (iii.) Pupininae, shell more or less lustrous, bluntly conical, lip with a channel above or below. Genera: Pupina (subg., Registoma, Callia, Streptaulus, Pupinella, Anaulus), Hybocystis (Fig. [205], p. 305), Cataulus, Coptochilus, Megalomastoma. (iv.) Cyclophorinae, shell turbinate or depressed, operculum corneous or calcareous. Genera: Alycaeus, Craspedopoma, Leptopoma, Lagochilus, Cyclophorus (Fig. [206], p. 306); including Diadema, Aulopoma, Ditropis, and others), Aperostoma (including Cyrtotoma and others), Cyathopoma, Pterocyclus (subg., Myxostoma, Spiraculum, Opisthoporus, and Rhiostoma (Fig. [180], p. 266), Cyclotus, Cyclosurus, and Strophostoma.
Fam. 13. Cyclostomatidae.—Ctenidium replaced by a pulmonary sac, tentacles obtuse, foot with a deep longitudinal median groove; central tooth, lateral, and first marginal more or less bluntly cusped, second marginal large, edge pectinate; shell variously spiral, spire usually elevated, aperture not quite circular; operculum generally with an external calcareous and an internal cartilaginoid lamina, rarely corneous. Terrestrial only. Cretaceous——. Genera: Cyclostoma (subg., Leonia, Tropidophora, Rochebrunia, Georgia, Otopoma, Lithidion, Revoilia), Cyclotopsis, Choanopoma (subg., Licina, Jamaicia, Ctenopoma, Diplopoma, Adamsiella), Cistula (subg., Chondropoma, Tudora), Omphalotropis (subg., Realia, Cyclomorpha), Hainesia, Acroptychia.
Fig. 274.—Cyclostoma campanulatum Pfr., Madagascar.
Fam. 14. Aciculidae.—Ctenidium replaced by a pulmonary sac, tentacles cylindrical, pointed at the end, eyes behind their base, foot long and narrow; central tooth and lateral very similar, pinched in at the sides, external marginal broad, edge finely pectinate; shell small, acuminate, with a blunt spire, operculum corneous. Terrestrial only. Tertiary——. Genus, Acicula (= Acme).
Fam. 15. Truncatellidae.—Ctenidium replaced by a pulmonary sac, proboscis very long, eyes sessile, behind the base of the tentacles, shell small, evenly cylindrical, apex truncated in the adult. Eocene——. Genera: Truncatella (subg., Taheitia, Blanfordia, and Tomichia), Geomelania (subg., Chittya and Blandiella), Cecina (?).
Fam. 16. Rissoidae.—Eyes at the external base of the tentacles, epipodium with filaments, operculigerous lobe with appendages; central tooth pleated at the basal angles, lateral large, bluntly multicuspid, marginals long, narrow, denticulate at the edge; shell small, acuminate, often elaborately sculptured, mouth entire or with a shallow canal, operculum corneous. Marine or brackish water. Jurassic——. Principal genera: Rissoa (subg., Folinia, Onoba, Alvania, Cingula, Nodulus, Anabathron, Fenella, Iravadia, and others), Scaliola (shell agglutinating fragments of sand, etc.), Rissoina (lip thickened, operculum with an apophysis as in Nerita), Barleeia, Paryphostoma (Eocene).
Fam. 17. Hydrobiidae.—Eyes at the outer base of the tentacles, penis behind the right tentacle, prominent, operculigerous lobe without filaments; radula rissoidan, central tooth often with basal denticulations; shell more or less acuminate, small, aperture entire, operculum corneous or calcareous. Brackish or fresh water. Jurassic——. Principal genera: Baicalia, with its various sub-genera (p. [290]); Pomatiopsis, Hydrobia, Bithynella, Micropyrgus (Tertiary), Pyrgula, Emmericia, Benedictia, Lithoglyphus, Tanganyicia, Limnotrochus (?), Jullienia, Pachydrobia, Potamopyrgus, Littorinida, Amnicola, Fluminicola (subg., Gillia, Somatogyrus), Bithynia, Fossarulus (Tertiary), Stenothyra.