Fam. 2. Trigoniidae.—Foot large, hatchet-shaped, with ventral disc; no byssus, mantle edge with ocelli; shell sub-trigonal, hinge-teeth few, strong; interior violet-nacreous. Devonian ——. Genera: Trigonia; Myophoria and Schizodus (Trias), Cyrtonotus (Devonian).
Fig. 301.—Trigonia pectinata Lam., Sydney, N.S.W.
Sub-order III. Mytilacea.—Mantle edges fused at one point, anal orifice distinct, anterior terminal adductor small, one aorta, branchiae with interfoliary junctions, genital glands penetrating the side of the mantle and opening by the side of the kidneys.
Fam. Mytilidae.—Byssus well developed, shell more or less equivalve, oval, broad; hinge-teeth evanescent. Devonian——. Principal genera: Mytilus?, Myalina, Septifer, Modiola, Lithodomus, Crenella, Dacrydium, Myrina, Idas, Modiolaria, Modiolarca.
Order III. Pseudolamellibranchiata
Mantle edges entirely open, foot little developed, anterior adductor usually aborted, branchial filaments reflected, with interlamellar junctions, which are sometimes vascular; genital glands opening into the kidneys or close to the apertures of the kidneys.
Fam. 1. Aviculidae.—Foot long, tongue-shaped, byssogenous apparatus well developed, branchiae concrescent with the mantle, adductor muscle sub-central, at times a small anterior adductor, siphons absent; shell usually inequivalve, dorsal margin straight, often very long, winged, lateral teeth much prolonged; structure of shell cellular, inside prismatic, outside nacreous. Palaeozoic ——. Principal genera: Avicula, including Meleagrina, Malleus; Vulsella (no wings or hinge-teeth); Perna, including Crenatula, Inoceramus (ligaments in a number of fossettes); Aucella and Monotis (Palaeozoic and Secondary); Pterinaea and Ambonychia (Palaeozoic); Pinna; Aviculopinna (Carboniferous).
Fig. 302.—Avicula heteroptera Lam., Australia, showing the inequivalve shell and byssal sinus (bs).