Monomyarian or with a very small anterior adductor. Mantle open; foot
rather small; branchiae folded; shell inequivalve.
Fam. 1. _Limidae._--Shell with auriculae. Foot digitiform, with
byssus. Borders of mantle with long and numerous tentacles. Gills
not united with mantle. _Lima_; members of this genus form a nest by
means of the byssus, or swim by clapping the valves of the shell
together. _Limaea._
Fam. 2. _Ostraeidae._--Foot much reduced and without byssus. Heart
usually on the ventral side of the rectum. Gills fused to the
mantle. Shell irregular, fixed in the young by the left and larger
valve. _Ostraea_; foot absent in the adult; edible and cultivated;
some species, as the British _O. edulis_, are hermaphrodite.
Fam. 3. _Eligmidae._--Extinct; Jurassic.
Fam. 4. _Pinnidae._--Shell elongated, truncated and gaping
posteriorly. Dimyarian, with a very small anterior adductor. Foot
with byssus. _Pinna_; British. _Cyrtopinna. Aviculopinna_; fossil,
Carboniferous and Permian. _Pinnigena_; Jurassic and Cretaceous.
_Atrina_; fossil and recent, from Carboniferous to present day.