Fam. 12. Ungulinidae.—Foot greatly elongated, vermiform, ending in a glandular enlargement. Ungulina. Diplodonta; British. Axinus; British.
Fam. 13. Cyrenellidae.—Two elongated, united, non-retractile siphons; freshwater. Cyrenella. Joanisiella.
Fam. 14. Tancrediidae.—Shell elongate, sub-triangular. Extinct. Tancredia; Trias to Cretaceous. Meekia; Cretaceous.
Fam. 15. Unicardiidae.—Shell sub-orbicular, nearly equilateral, with concentric striae. Extinct, Carboniferous to Cretaceous. Unicardium. Scaldia. Pseudedmondia.
Fam. 16. Leptonidae.—Shell thin; no siphons; foot long and byssiferous; marine; hermaphrodite and incubatory. Kellya; British. Lepton; commensal with the Crustacean Gebia; British. Erycina; Tertiary. Pythina. Scacchia. Sportella. Cyamium.
Fam. 17. Galeommidae.—Mantle reflected over shell; shell thin, gaping; adductors much reduced. Galeomma; British. Scintilla. Hindsiella. Ephippodonta; commensal with shrimp Axius. The three following genera with an internal shell probably belong to this family:—Chlamydoconcha. Scioberetia; commensal with a Spatangid. Entovalva; parasitic in Synapta.
Fam. 18. Kellyellidae.—Shell ovoid; anal aperture with very short siphon; foot elongated. Kellyella. Turtonia; British. Allopagus; Eocene. Lutetia; Eocene.
| Fig. 28.—Lateral view of a Mactra, the right valve of the shell and right mantle-flap removed, and the siphons retracted. (From Gegenbaur.) |
| br, br′, Outer and inner gill-plates. t, Labial tentacle. ta, tr, Upper and lower siphons. ms, Siphonal muscle of the mantle-flap. ma, Anterior adductor muscle. mp, Posterior adductor muscle. p, Foot. c, Umbo. |
Fam. 19. Cyrenidae.—Two siphons, more or less united, with papillose orifices; pallial line with a sinus; freshwater. Cyrena. Corbicula. Batissa. Velorita. Galatea. Fischeria.
Fam. 20. Cycladidae.—One siphon or two free siphons with simple orifices; pallial line simple; hermaphrodite, embryos incubated in external gill-plate; freshwater, Cyclas; British. Pisidium; British.