Hermaphrodite, the ovaries and testes distinct, with separate
apertures. Foot rather small. Mantle frequently presents a fourth
orifice. External gill-plate directed dorsally and without reflected
lamella. Hinge without teeth.
Fam. 1. _Thracidae._--Mantle with a fourth aperture; siphons long,
quite separate, completely retractile and invertible. _Thracia_;
British. _Asthenothaerus._
Fam. 2. _Periplomidae._--Siphons separate, naked, completely
retractile but not invertible. _Periploma. Cochlodesma. Tyleria._
Fam. 3. _Anatinidae._--Siphons long, united, covered by a chitinous
sheath, not completely retractile. _Anatina. Plectomya_; Jurassic
and Cretaceous.
Fam. 4. _Pholadomyidae._--Mantle with fourth aperture; siphons very
long, completely united, naked, incompletely retractile; foot small,
with posterior appendage. _Pholadomya._
Fam. 5. _Arcomyidae._--Extinct; Secondary and Tertiary. _Arcomya._
_Goniomya._
Fam. 6. _Pholadellidae._--Extinct; Palaeozoic. _Pholadella.
Phytimya. Allorisma._
Fam. 7. _Pleuromyidae._--Extinct; Secondary. _Pleuromya. Gresslya._
_Ceromya._
Fam. 8. _Pandoridae._--Shell thin, inequivalve, free; ligament
internal; siphons very short. _Pandora_; British. _Coelodon._
_Clidiophora._
Fam. 9. _Myochamidae._--Shell very inequivalve, solid, with a
pallial sinus; siphons short; foot small. _Myochama. Myodora._
Fam. 10. _Chamostraeidae._--A fourth pallial aperture present; pedal
aperture small; siphons very short and separate; shell fixed by the
right valve, irregular. _Chamostraea._
Fam. 11. _Clavagellidae._--Pedal aperture very small, foot
rudimentary; valves continued backwards into a calcareous tube
secreted by the siphons. _Clavagella. Brechites (Aspergillum)._
Fam. 12. _Lyonsiidae._--Foot byssiferous; siphons short, invertible.
_Lyonsia_; British. _Entodesma. Mytilimeria._
Fam. 13. _Verticordiidae._--Siphons short, gills papillose; foot
small; shell globular. Many species abyssal. _Verlicordia._
_Euciroa. Lyonsiella. Halicardia._