Mantle closed to a considerable extent; siphons well developed; gills
much folded and frequently prolonged into the branchial siphon. Foot
compressed and generally byssiferous. Shell gaping, with a pallial
sinus.
Fam. 1. _Psammobiidae._--Siphons very long and quite separate; foot
large; shell oval, elongated, ligament external. _Psammobia_;
British. _Sanguinolaria. Asaphis. Elizia. Solenotellina._
Fam. 2. _Myidae._--Siphons united for the greater part of their
length, and with a circlet of tentacles near their extremities; foot
reduced; shell gaping; ligament internal. _Mya_; British. _Sphenia_;
British. _Tugonia. Platyodon. Cryptomya._
Fam. 3. _Corbulidae._--Shell sub-trigonal, inequivalve; pallial
sinus shallow; siphons short, united, completely retractile; foot
large, pointed, often byssiferous. _Corbulomya. Paramya. Erodona_
and _Himella_ are fluviatile forms from South America.
Fam. 4. _Lutrariidae._--Mantle extensively closed; a fourth pallial
aperture behind the foot; siphons long and united; shell elongated,
a spoon-shaped projection for the ligament on each valve.
_Lutraria_; British. _Tresus. Standella._
Fam. 5. _Solenidae._--Elongated burrowing forms; foot cylindrical,
powerful, without byssus; shell long, truncated and gaping at each
end. _Solenocurtus_; British. _Tagelus_; estuarine. _Ceratisolen_;
British. _Cultellus_; British. _Siliqua. Solen_; British. _Ensis_;
British.
Fam. 6. _Saxicavidae._--Mantle extensively closed, with a small
pedal orifice; siphons long, united, covered by a chitinous sheath;
gills prolonged into the branchial siphon; foot small; shell gaping.
_Saxicava_; British. _Glycimeris. Cyrtodaria._
Fam. 7. _Gastrochaenidae._--Shell thin, gaping widely at the
posterior end; anterior adductor much reduced; mantle extensively
closed; siphons long, united. _Gastrochaena_; British. _Fistulana._