Early Ordovician – Recent
Arcoida is an extant order of facultatively mobile semi-infaunal suspension feeders
Characteristics of the Order
- Two adductor muscles, thus the muscle scars, are of approximately the same size (isomyarian)
- Shell completely aragonitic (likely not preserved in Ordovician strata)
- Shells of equal size and form
Waller & Yochelson (1978):
- Isomyarian or anisomyarian Bivalvia having an outer fold and one or two inner folds on the mantle edge, the outer fold muscular and generally bearing eyespots, the inner folds lacking fusion ventral to the excurrent region. Shell completely aragonitic, with outermost layer simple crossed-lamellar and complex-prismatic in structure or only the former; inner layer complex cross-lamellar. Ligament duplivincular.
Treatise on Invertebrate Paleontology, Part N, Mollusca 6 (1 of 3) (1969):
- Arks and their near relatives; sedentary, with free or byssate adults; characteristically isomyarian and equivalved with circular to trapezoidal shells; interior shell microstructure cross-lamellar; gills fillibranchiate in living forms; in most groups, dorsal margin bears narrow to broad, flat cardinal areas that rise above hinge axis.