Arcoida (Arcoidea & Limopsoidea) (500 - 5000m)
Ligament on a small triangular resilium between beaks (alivincular) [c1], shell obliquely oval, hairy [c2] and adductor scars very unequal [c3]. [Limopsoidea]
Ligament as a series of bands (duplivincular) over the dorsal area or only on the posterior part [c4], shell rhomboidal to trapezoidal, adductor scars approximately equal in size [c5]. [Arcoidea]
Inner margin crenulate [c6].
Inner margin smooth [c7].
Obliquely oval shell, hairs adpressed [c8], posterior ventral crenulae larger.
Oval shell, bristles erect [c9], crenulae of similar size.
Relatively thick shells with impressed muscle scars, outer shelf and upper slope [c10].
Relatively thin shells with indistinct scars, abyssal rise and abyssal plain [c11].
Shells angular wedge -shaped, greatly reduced anteriorly, with a distinct byssal sulcus [c12].
Shells roundly wedge-shaped to subcircular, with a wide edentulous space below beaks [c13]. [Bathyarca]
Sculpture nodulose [c14], (upper slope).
Sculpture weak, primarily commarginal [c15] (abyssal).
Shell subcircular globose, distinctly inequivalve [c16].
Shell longer than high, roundly wedge-shaped.
Shell distinctly longer than high, sculpture of riblets with scales [c17].
Shell length a little greater than height, sculpture weak never with scales but with periostracal hairs.
Shell fragile, length = height, sculpture indistinct, bristles prominent [c18] (Arctic).
Shell longer than high, sculpture reticulate, bristles weak [c19].