*Images in development
Arcoida (Arcoidea & Limopsoidea)
Ligament on a small triangular resilium between beaks (alivincular), shell obliquely oval, hairy and adductor scars very unequal = Limopsoidea
Ligament as a series of bands (duplivincular) over the dorsal area or only on the posterior part, shell subcircular, rhomboidal to trapezoidal, adductor scars approximately equal in size = Arcoidea
Obliquely oval shell, hairs adpressed, posterior ventral crenulae larger
Limopsis minuta
Relatively thick shells with impressed muscle scars, outer shelf and upper slope
Limopsis aurita
Relatively thin shells with indistincr scars, abyssal rise and abyssal plain
Limopsis tenella
Rhomboidal to trapezoidal shells
= Arcidae/Noetiidae
Rhomboidal to trapezoidal ligament of v-shaped bands between or only behind beaks = Arcidae
Trapezoidal (often irregular) with numerous chevrons on a wide dorsal area
Rhomboidal or wedge shaped, ligament of few bands behind beaks
Trapezoidal often irregular, adductor scars with myophoric ridges, ligament of numerous fine chevrons
Arca tetragona
Regularly trapezoidal, chevrons few and regular, adductor scars lacking myophoric ridges (aliens on marine litter)
Elongate trapezoidal with braod radial ribs, posterior margin indented, periostracum of frills
Arca zebra
Trapezoidal with narrow riblets, posterior margin truncate, periostruc of erect bristles those on the posterior carina well developed
Arca imbricata
Shells angular wedge -shaped, greatly reduced anteriorly, with a distinct byssal sulcus
Shells roundly wedge-shaped to subcircular, with a wide edentulous space below beaks
= Bathyarca
Shell longer than high, rhomboidal (oblong) to roundly wedge-shaped
Shell roundly wedge-shaped
Shell distinctly longer than high, sculpture of riblets with scales, more prominent towards margins
Bathyarca philippiana
Shell length equal or a little greater than height, sculpture weakly reticulate (radial lines over commarginal ridges), never with scales
Shell fragile, length=height, sculpture indistinct, bristles prominent (deep water, Arctic)
Bathyarca frielei
Shell longer than high, sculpture reticulate, bristles weak (outer shelf and upper slope everywhere)
Bathyarca pectunculoides
