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The Philobryidae (Eocene-Recent; order Arcoida) are redefined to include nine genera: Adacnarca, Aupouria, Cosa, Gratis, Limarca, Limopsilla, Lissarca, Neocardia, and Philobrya. Philobryids are generally small (2-10 mm), mytiliform, and have a ligament pit that lies between two series of interlocking denticles. Philobryids are most common and widely distributed in the Southern Hemisphere. They are epibyssate but also are efficient epifaunal crawlers. The Limopsidae (Jurassic-Recent; order Arcoida) are redefined to include two genera: Empleconia and Limopsis. Limopsids are small (10-60 mm), ovate, and have a ligament pit that lies in a dorsal area above numerous teeth. Limopsids are endobyssate and are cosmopolitan at shelf depths. Unlike the gill of other arcoids, the philobryid gill is composed of short, stubby filaments and has few ciliary junctures. These features of the philobryid gill represent adaptations for cleansing, strengthening the gill, and viviparity. Some philobryids have an anterior inhalant area. The presence of this feature in taxonomically remote pelecypod groups and in groups of Recent, highly specialized pelecypods shows that, contrary to prevalent opinion, its presence does not necessarily indicate primitiveness or taxonomic affinity. Morphological, distributional, and temporal evidence suggests that limopsids probably arose from grammatodonts and in turn gave rise to glycymerids and philobryids. Neoteny may have played a role in philobryid and limopsid evolution.