Chapter XXI: 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).