Vlastidae.-Shell very inequivalve; hinge without teeth.
Spondylidae.-Shell very inequivalve, fixed by the right valve which is the larger.
Asymmetrical, inequivalve, fixed, with extensive pallial sutures; no siphons.
Caprinidae.-Shell inequivalve; fixed valve spiral or conical; free valve coiled or spiral; Cretaceous.
Monopleuridae.-Shell very inequivalve; fixed valve conical or spiral; free valve operculiform; Cretaceous.
- Shell sub-trigonal, inequivalve; pallial sinus shallow; siphons short, united, completely retractile; foot large, pointed, often byssiferous.
- Shell thin, inequivalve, free; ligament internal; siphons very short.
- Shell very inequivalve, solid, with a pallial sinus; siphons short; foot small.