Fam. 7. Gastrochaenidae.—Shell thin, gaping widely at the posterior end; anterior adductor much reduced; mantle extensively closed; siphons long, united. Gastrochaena; British. Fistulana.
Sub-order VIII.—Adesmacea.
Ligament wanting; shell gaping, with a styloid apophysis in the umbonal cavities. Gills prolonged into the branchial siphon. Mantle largely closed, siphons long, united. Foot short, truncated, discoid, without byssus.
Fam. 1. Pholadidae.—Shell containing all the organs; heart traversed by the rectum; two aortae. Shell with a pallial sinus; dorsal region protected by accessory plates. Pholas; British. Pholadidea; British. Jouannetia. Xylophaga; British. Martesia.
Fam. 2. Teredinidae.—Shell globular, covering only a small portion of the vermiform body; heart on ventral side of rectum; a single aorta; siphons long, united and furnished with two posterior calcareous “pallets.” Teredo; British. Xylotrya.
Sub-order IX.—Anatinacea.
Hermaphrodite, the ovaries and testes distinct, with separate apertures. Foot rather small. Mantle frequently presents a fourth orifice. External gill-plate directed dorsally and without reflected lamella. Hinge without teeth.
Fam. 1. Thracidae.—Mantle with a fourth aperture; siphons long, quite separate, completely retractile and invertible. Thracia; British. Asthenothaerus.
Fam. 2. Periplomidae.—Siphons separate, naked, completely retractile but not invertible. Periploma. Cochlodesma. Tyleria.
Fam. 3. Anatinidae.—Siphons long, united, covered by a chitinous sheath, not completely retractile. Anatina. Plectomya; Jurassic and Cretaceous.