Fam. 6. Aetheriidae.—Anal orifice complete, foot absent, labial palps large; shell irregular, free or fixed, no hinge-teeth. Fluviatile, recent only. Genera: Aetheria, Mülleria, Bartlettia.
Fam. 7. Unionidae.—Foot large and thick, no byssus, anal siphon short, branchial orifice complete or not, siphon present or absent, embryo of certain groups passing through a glochidium stage (p. [146]); shell equivalve, sometimes very thick, nacreous within, hinge variable. Fluviatile. Jurassic——. Principal genera: Unio (subg. Arconaia), Monocondylaea, Pseudodon, Anodonta, Solenaia, Mycetopus, Mutela, Spatha, Hyria, Castalia, Leila.
Fam. 8. Dreissensiidae.—Both siphons prominent, foot tongue-shaped, byssiferous; shell mytiliform, with small internal septum. Genera: Dreissensia; Dreissensiomya (Tertiary). The common Dreissensia polymorpha Pall. was distributed over large parts of Europe in later Tertiary times. From unknown causes it died out, and has during the past two hundred years been regaining its position, migrating N. and W. from its original habitat, the Caspian, by the Volga and its Oka confluent.
Fam. 9. Modiolopsidae.—Shell mytiliform, ligament exterior, hinge-teeth small, rather numerous. Palaeozoic——. Principal genera: Modiolopsis, Cyrtodonta, Mytilops, Ptychodesma.
Fam. 10. Lucinidae.—Anal orifice sometimes with a siphon, branchial orifice complete or not, sometimes a single branchia; foot very long, vermiform, no byssus, anterior adductor long; shell rounded, equivalve, blanched, hinge with two cardinals and two laterals in each valve, sometimes toothless, ligament more or less internal. Silurian——. Principal genera: Lucina, Corbis, Axinus, Diplodonta, Montacuta.
Fam. 11. Ungulinidae.—Anal orifice complete, foot vermiform, no byssus, two branchiae; shell equivalve, subcircular, hinge-teeth variable, no laterals, adductor impressions long, continuing the pallial line. Tertiary——. Single genus, Ungulina.
Fam. 12. Unicardiidae.—Shell equivalve, round or oval, cardinal shelf large, a single cardinal in each valve, ligament external. Carboniferous—Cretaceous. Genera: Unicardium, Scaldia, Pseudedmondia.
Fam. 13. Kellyellidae.—Anal siphon prolonged, no marked branchial orifice; shell very small, oval or round, anterior lateral very strong, under the cardinal. Eocene——. Genera: Kellyella; Allopagus and Lutetia (Tertiary), Turtonia.
Fam. 14. Erycinidae.—Mantle edges with three apertures, branchial orifice on the buccal margin, foot long, broadened, with a byssus, animal usually viviparous. Tertiary——. Genera: Erycina, Kellia, Pythina, Lasaea, Lepton.
Fam. 15. Galeommidae.—Mantle edges more or less reflected over the shell, apertures and foot as in Erycinidae; shell thin, equilateral, hinge with few teeth or none. Tertiary——. Genera: Galeomma, Scintilla, Sportella, Chlamydoconcha, Hindsiella, Ephippodonta (Fig. [32], p. 81).