The diversity and adaptive radiations of modern Arcoidea, here considered to contain the families Arcidae, Noetiidae, Cucullaeidae, and Glycymerididae, are reviewed. Most fall into either epibyssate or endobyssate life habits with only the Glycymerididae living as free burrowers. The phenetic characters of the families within the Arcoida are reviewed and the families are shown to be supported by very few synapomorphic characters. Homoplasy is shown to be widespread and is illustrated in a series of discussions on the ligament, epibyssate-endobyssate radiations, and possible parallelism within genera, and in a review of arcoid anatomical characters. Previously published molecular data are reviewed and these support the inclusion of the Glycymerididae in the Arcoidea. They also indicate, however, that polyphyly is probably widespread at the subfamily level.

The Thyasiroidea collected from the mud volcanoes of the Gulf of Cadiz are reviewed. Of the seven species identified only one, Thyasira vulcolutre n. sp., is closely associated with a chemosynthetic setting. This species has anatomical features typical of chemosymbiotic taxa and is compared with T. sarsi (Philippi, 1845), T. southwardae Oliver & Holmes, 2006, T. oleophila Clarke, 1989 and T. methanophila Oliver & Sellanes, 2005. The other six, Thyasira (Parathyasira) granulosa (Monterosato, 1874), Thyasira tortuosa (Jeffreys, 1881), Thyasira obsoleta (Verrill & Bush, 1898), Axinulus croulinensis (Jeffreys, 1847), Mendicula ferruginosa (Forbes, 1844) and Leptaxinus minutus Verrill & Bush 1898, are previously known from typical deep-water benthic settings. Of these only A. croulinensis is known to harbour chemosymbionts although the gill anatomy of T. tortuosa suggests that it might also be chemosymbiotic. Thyasira vulcolutre is restricted to active seeps but there is no pattern in the distribution of the other species.

A new species of Thyasira (T. methanophila n. sp.) is described from a methane seepage area off Concepción, Chile (~36ºS) at bathyal depths. This species shows affinity with other southern ocean species placed in the subgenus Maorithyas. The gross anatomy of the type species of Maorithyas, M. marama Fleming, 1950, is described for the first time. There are no gross anatomical differences in the ctendia, foot and gut, between the above taxa, which suggest that there are no differences anatomically between species living at seeps and species living in other environments. Furthermore, wider comparison of the shells suggests that the subgenus Maorithyas cannot be supported, as the differential characters grade into those of Thyasira.        Scanning electron microscopy of the ctenidium, of T. methanophila, revealed filaments with closely packed bacteriocytes each with dense aggregations of small rod shaped bacteria each about 0.5 mm in length. The bivalve is assumed to be highly dependent on the symbiotic relationship.        A second thyasirid was recorded from worn valves only and is assigned to the genus Conchocele. The material is too fragmentary to describe but was compared with C. bisecta and C. novaeguinensis.

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