Fam. 5. Ceromyidae.—Shell inequivalve, large, heart- or wedge-shaped, hinge toothless, ligament internal in one valve, external in the other. Secondary strata——. Genera: Ceromya, Gresslya.
Fam. 6. Arcomyidae.—Shell equivalve, thin, surface finely granulated, hinge toothless, cardinal edge dentiform, ligament external. Secondary and Tertiary strata——. Genera: Arcomya, Goniomya, Pleuromya, Machomya.
Fam. 7. Anatinidae.—A fourth (?byssal) pallial orifice, siphons long, separate or fused; shell thin, sometimes inequivalve, exterior often granulose, ligament often with ossicle, hinge toothless or with lamellae. Jurassic——. Genera: Anatina, Plectomya (Secondary), Periploma, Cochlodesma, Thracia, Tyleria, Alicia, Asthenothaerus.
Fam. 8. Grammysiidae.—Shell equivalve, oval, ligament external, cardinal margin straight, toothless, pallial line complete. Palaeozoic——. Principal genus, Grammysia, with many other genera of toothless hinge, but whose exact affinities are uncertain.
Fam. 9. Praecardiidae.—Shell thin, equivalve or not, radiately ribbed, margins dentated, sub-umbonal area as in Arca, hinge toothless. Palaeozoic——. Principal genus, Praecardium.
Fam. 10. Pholadomyidae.—A fourth pallial orifice, siphons very long, united, foot small; shell thin, equivalve, with radiating ribs, ligament external, hinge toothless, pallial line sinuate. Jurassic——. Single genus, Pholadomya.
Fam. 11. Clavagellidae.—Foot rudimentary, siphons long, united, contained in a long calcareous tube; shell small, one or both valves soldered in the tube, tube with a centrally fissured disc at the anterior end, more or less frilled at the posterior end. Cretaceous——. Genera: Clavagella, Brechites (= Aspergillum).
Order V. Septibranchiata
Mantle edges united at three points, branchiae replaced by a muscular septum extending from the anterior adductor to the point of separation of the siphons, septum with symmetrical orifices.
Fam. 1. Poromyidae.—Branchial septum with groups of lamellae between the orifices, labial palps large, foot long and narrow, siphons short, papillose, separated, animal hermaphrodite; shell small, slightly inequivalve, rounded or oblong, nacreous within. Eocene——. Genera: Poromya, Silenia.