Protobranchia (500 - 5000m)
Shell height greater than or equal to length, subcircular to trigonal [c1, c2, c3], ligament internal in an anteriorly directed (opisthodetic) chondrophore [Nuculoidea & Pristiglomoidea]
Shell longer than high, frequently subrostrate or rostrate, [c4, c5, c6] ligament internal in a shallow resilifer or external. [Nuculanoidea]
Shell subcircular or obliquely oval, hinge with few posterior teeth subparallel to hinge plate [c7].[Pristiglomatoidea]
Shell triangular to subovate, hinge with numerous, prominent erect teeth [c.8] [Nuculoidea]
Shell obliquely oval [c9]
Internal margin denticulate [c11]
Internal margin smooth [c12]
Triangular distinctly higher than long [c13], marked decussate sculpture, commarginal ridges becoming stronger towards margin. (bathyal)
Shell subovate [c14], prominent larval shell, cancellate sculpture
Shells distinctly higher than long, triangular or rhomboidal
Shells longer than high subtriangular or subovate
Shell triangular anterior and posterior equally inclined [c15].
Shell rhomboidal, umbos inflated [c16].
Shell subovate, posterior truncated [c17], surface microscopically granular [c18]
Shell subovate, anterior slope subacute [c19]
Hinge teeth few, elongate, subparallel with hinge plate [c20].
Hinge teeth pointed erect
Shells subrostrate with a posterior ventral sinus
Shells rostrate or subovate to oval
Shells with a sharply pointed dorsally situated posterior, ligament both interior and exterior, gut loop multi-coiled.
Shells with a pointed, medially or downwardly angled posterior, ligament interior, gut loop simple or multi-coiled.
Posterior much deeper than anterior, posterior below rostrum bulging.
Posterior a little deeper than anterior, sulcus below rostrum weak.
Rostrum acute. Posterior dorsal margin sloping (bathyal)
Shell thick becoming tumid, sculpture of fine commarginal ridges.
Shells normal thickness, sculpture of weak radial and commarginal ridges, periostracum pustulose.
Minute (<1.5mm), rounded tumid shells
Outline elliptical with commarginal ridges at margins, teeth prominent
Outline subcircular (southern, bathyal)
External ligament visible close to beaks
Ligament internal not visible in joined valves.
Shell oblong, posterior broad, roundly truncated
Shell subovate to elliptical, posterior pointed
Subovate, robust shells with commarginal ridges
Elliptical, fragile smooth shells
Robust, narrowly rostrate with prominent commarginal sculpture
Thin shells with weak sculpture, subovate to oblong.