Section Ii. Azygobranchiata.--One Ctenidium (the Left) Present.

FAM. 1. Cocculinidae.--A single cervical ctenidium, foot broad, no eyes, shell patelliform, with caducous spire. Single genus, Cocculina. Deep water.

FAM. 2. Stomatellidae.--A single (left) ctenidium, front third free, shell nacreous, spiral or patelliform, depressed, last whorl large. Jurassic----. Genera: Stomatella (subg. Synaptocochlea, Niphonia), shell depressed, spirally ribbed, spire short, operculum present; Phaneta, fluviatile only, shell trochiform, imperforate, last whorl keeled, sinuate in front; Stomatia, spire short, surface tubercled or keeled, no operculum; Gena, shell haliotis-shaped, surface smooth, aperture very large; Broderipia, shell patelliform, spiral apex often lost.

FAM. 3. Cyclostrematidae.--Tentacles ciliated, thread-like, snout bilobed, foot truncated in front, angles produced into a filament, shell depressed, umbilicated, not nacreous. Eocene----. Principal genera: Cyclostrema, Teinostoma, Vitrinella.

FAM. 4. Liotiidae.--Epipodial line with a lobe behind each eye-peduncle, shell solid, trochiform, longitudinally ribbed or trellised, aperture round, operculum multispiral, hispid, corneous, with a calcareous layer. Silurian----. Principal genera: Liotia, Craspedostoma (Silurian), Crossostoma (Jurassic).

FAM. 5. Trochidae.--Snout short, broad, frontal lobes often present, epipodial line furnished with cirrhi; shell nacreous, variously spiral, operculum corneous, multispiral, nucleus central (Fig. 182, p. 268). Silurian----. (i.) Trochinae.--Frontal lobes present, lateral teeth (= side centrals) 5 only, no jaws, peristome incomplete. Principal genera: Trochus (subg. Cardinalia, Tectus, Infundibulum, Clanculus), Monodonta (subg. Diloma), Cantharidus (subg. Bankivia, Thalotia), Gaza (subg. Microgaza), Callogaza, Bembix, Chlorostoma. (ii.) Gibbulinae.--Frontal lobes and jaws present, laterals often more than 5, peristome incomplete. Principal genera: Gibbula (subg. Monilia, Aphanotrochus, Enida), Minolia, Circulus, Trochiscus, Livona, Photinula, Margarita, Solariella, Calliostoma, Turcica, Basilissa, Euchelus (subg. Olivia, Perrinia). (iii.) Delphinulinae.--No frontal lobes, jaws present; shell solid, surface spirally lirate, scaly, spinose, umbilicate, peristome continuous. Single genus, Delphinula. (iv.) Umboniinae.--Eyes pedunculate, left tentacle attached to a frontal appendage, mantle reflected over edge of aperture, lateral teeth 6 on each side; shell polished, peristome incomplete, umbilicus generally closed by a callosity. Principal genera: Umbonium, Ethalia, Isanda, Camitia, Umbonella, Chrysostoma.

FAM. 6. Turbinidae.--Epipodial line with slender cirrhi, snout broad, short, eyes pedunculate at outer base of tentacles, a frontal veil between tentacles; shell turbinate, solid, aperture continuous, operculum solid, calcareous, usually paucispiral, convex exteriorly (Fig. 182, p. 268). Silurian----. (i.) Phasianellinae.--Shell bulimoid, polished, not nacreous, coloured in patterns, aperture oval. Single genus, Phasianella (Fig. 271). (ii.) Turbininae.--Shell very solid, nacreous within, aperture circular or long oval. Principal genera, Turbo, whorls rounded above and below, spines, if present, becoming more prominent with age, operculum smooth or granulose, nucleus sub-central; subg. Callopoma, Ninella, Marmorostoma, Sarmaticus, Prisogaster; Astralium, whorls flattened above and below, spines, if present, becoming less prominent with age, operculum oblong, often excavated at centre, last whorl large, nucleus marginal or sub-marginal; subg. Lithopoma, Imperator, Guildfordia, Bolma, Cyclocantha, Uvanilla, Cookia, Pomaulax, Pachypoma. (iii.) Cyclonematinae.--Shell nacreous, umbilicate, operculum conical outside, whorls scalariform. Principal genera: Cyclonema, Horiostoma (?), Amberleya (Silurian to Lias). (iv.) Leptothyrinae.--Shell small, solid, depressed, operculum nearly flat, nucleus sub-central. Genera: Leptothyra, Collonia (?).

FAM. 7. Neritopsidae.--Tentacles wide apart, long, eyes on short peduncles at the outer base; shell solid, neritiform or naticoid, aperture semi-lunar or oval; operculum (Fig. 183, p. 269) thick, calcareous, non-spiral, exterior face smooth, interior face divided into two unequal parts, with a broad median appendage. Devonian----. Principal genera: Neritopsis (one recent species), Naticopsis (Devonian to Miocene).

FAM. 8. Macluritidae.--Shell discoidal, whorls few, longitudinally grooved behind, right side convex, deeply umbilicated, left side flat; operculum very thick, nucleus excentrical, internal face with two apophyses, one very large. The general appearance is more that of an inequivalve bivalve, such as Requienia, than of a spiral gasteropod. Palaeozoic----. Single genus, Maclurea.

FAM. 9. Neritidae.--Snout short, tentacles long, eyes pedunculate at their outer base, branchia triangular, free at the front end, epipodium without cirrhi, penis near the right tentacle; shell solid, imperforate, turbinate to almost patelliform, spire short, internal partitions absorbed (p. 168), columellar region broad, edge simple or dentate, operculum calcareous, spiral or non-spiral, with prominent apophyses on the interior face, one of which locks behind the columellar lip. Jurassic----. Principal genera: Nerita (Fig. 13, p. 17); Neritina (chiefly brackish water and fluviatile), sub-genus Clithon, usually coronated with spines; Velates (Tertiary), Neritoma (Jurassic), Deianira (Cretaceous), Septaria (= Navicella), shell more or less narrowly patelliform, with terminal apex, aperture very large, with a broad columellar septum, operculum too small for the aperture, more or less covered by the integument of the foot; fluviatile only; Pileolus (Jurassic to Cretaceous).

FAM. 10. Hydrocenidae.--Branchia replaced by a pulmonary chamber, eyes at the outer base of the tentacles, marginals of the radula very oblique, centrals often wanting; shell small, conical, whorls convex, operculum calcareous, with a prominent apophysis. Recent. Principal genera: Hydrocena, Georissa.

FAM. 11. Helicinidae.--Branchia replaced by a pulmonary chamber, heart with one auricle; shell globular, with a short spire, internal partitions absorbed; operculum without apophysis. Carboniferous----. Principal genera: Helicina (Fig. 18B, p. 21; subg. Alcadia, Schasicheila, Heudeia, Calybium), Eutrochatella (subg. Lucidella), Stoastoma, Bourcieria, Dawsonella (Carboniferous).

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. 18C, 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).