mactracea

from The Century Dictionary.

• noun A family of acephalous or bivalve mollusks, comprising the genera Mactra, Lutraria, Crassatella, Erycina, Ungulina, Solemya, and Amphidesma, and scattered in several different families.
• noun Now a suborder or superfamily of bivalves, including only the family Mactridæ and related forms.