Mytiloida (phylum Mollusca, class Bivalvia) An order of epifaunal, byssate bivalves which have an equivalve, but highly non-equilaterally shaped, shell with a prismatic-nacreous microstructure. They have a dysodont dentition, the ligament is parivincular and opisthodetic, and the gills are eulamellibranchiate and filibranchiate. Mytiloida have an anisomyarian musculature. The pallial line is complete, the siphons poorly developed. They first appeared in the Devonian.

Mytiloida (phylum Mollusca, class *Bivalvia) Order of epifaunal, byssate bivalves which have an equivalve shell that has a highly non-equilateral shape and a prismatic-nacreous microstructure (e.g. Mytilus edulis, the common mussel). They have a dysodont dentition, the ligament is elongate and opisthodetic. Mytiloida have an anisomyarian musculature (i.e. the posterior adductor muscle and its scar are larger than the anterior adductor muscle). The pallial line is complete, the siphons poorly developed. They first appeared in the Devonian. See MUSCLE SCAR.