Monomyarian, with open mantle. Gills folded and the filaments at
summits and bases of the folds are different from the others. Gonads
contained in the visceral mass and generally open into renal cavities.
Foot usually rudimentary.
Fam. 1. _Vulsellidae._--Shell high; hinge toothless; foot without
byssus. _Vulsella._
Fam. 2. _Aviculidae._--Shell very inequilateral; cardinal border
straight with two auriculae, the posterior the longer. Foot with a
very stout byssus. Gills fused to the mantle. _Avicula_; British.
_Meleagrina._ Pearls are obtained from a species of this genus in
the Persian Gulf, Indian Ocean, &c. _Malleus._ Several extinct
genera.
Fam. 3. _Prasinidae._--Shell inequilateral, with anterior umbones
and prominent anterior auricula; cardinal border arched. _Prasina._
Fam. 4. _Pterineidae._--Extinct; Palaeozoic.
Fam. 5. _Lunulicardiidae._--Extinct; Silurian and Devonian.
Fam. 6. _Conocardiidae._--Extinct; Silurian to Carboniferous.
Fam. 7. _Ambonychiidae._--Extinct; Silurian and Devonian. The last
two families are dimyarian, with small anterior adductor.
Fam. 8. _Myalinidae._--Extinct; Silurian to Cretaceous; adductors
sub-equal.
Fam. 9. _Amussiidae._--Shell orbicular, smooth externally with
radiating costae internally. Gills without interlamellar junctions.
_Amussium._
Fam. 10. _Spondylidae._--Shell very inequivalve, fixed by the right
valve which is the larger. No byssus. _Spondylus_; shell with spiny
ribs, adherent by the spines. _Plicatula._
Fam. 11. _Pectinidae._--Shell with radiating ribs; dorsal border
with two auriculae. Foot byssiferous. Mantle borders with well
developed eyes. _Pecten_; shell orbicular, with equal auriculae;
without a byssal sinus; British. _Chlamys_; anterior auricula the
larger and with a byssal sinus; British. _Pedum. Hinnites.
Pseudamussium. Camptonectes. Hyalopecten_; abyssal.