Fam. 6.—Fissurellidae. Shell conical; slit or hole in anterior part of mantle; two symmetrical ctenidia; no operculum. Emarginula, mantle and shell with a slit, British. Scutum, mantle split anteriorly and reflected over shell, which has no slit. Puncturella, mantle and shell with a foramen in front of the apex, British. Fissurella, mantle and shell perforated at apex, British.

Fam. 7.—Cocculinidae. Shell conical, symmetrical, without slit or perforation. Cocculina, abyssal.

Fam. 8.—Trochidae. Shell spirally coiled; a single ctenidium; eyes perforated; a horny operculum; lobes between the tentacles. Trochus, shell umbilicated, spire pointed and prominent, British. Monodonta, no jaws, spire not prominent, no umbilicus, columella toothed. Gibbula, with jaws, three pairs of epipodial cirri without pigment spots at their bases, British. Margarita, five to seven pairs of epipodial cirri with a pigment spot at base of each.

Fig. 16.—Scutum,seen from the pedal surface. (Lankester.)

o, Mouth.

T, Cephalic tentacle.

br, One of the two symmetrical gills placed on the neck.

Fig. 17.—Dorsal aspect of a specimen of Fissurella fromwhich the shell has been removed, whilst the anterior area of the mantle-skirt hasbeen longitudinally slit and its sides reflected. (Lankester.)

a, Cephalic tentacle.

b, Foot.

d, Left (archaic right) gill-plume.

e, Reflected mantle-flap.

fi, The fissure or hole in the mantle-flap traversed by the longitudinal incision.

f, Right (archaic left) nephridium’s aperture.

g, Anus.

h, Left (archaic right) aperture of nephridium.

p, Snout.

Fam. 9.—Stomatellidae. Spire of shell much reduced; a single ctenidium. Stomatella, foot truncated posteriorly, an operculum present, no epipodial tentacles. Gena, foot elongated posteriorly, no operculum.

Fam. 10.—Delphinulidae. Shell spirally coiled; operculum horny; intertentacular lobes absent. Delphinula.

Fam. 11.—Liotiidae, shell globular, margin of aperture thickened. Liotia.

Fam. 12.—Cyclostrematidae. Shell flattened, umbilicated; foot anteriorly truncated with angles produced into lobes. Cyclostrema. Teinostoma.

Fam. 13.—Trochonematidae. All extinct, Cambrian to Cretaceous.

Fam. 14.—Turbinidae. Shell spirally coiled; epipodial tentacles present; operculum thick and calcareous. Turbo. Astralium. Molleria. Cyclonema.