Mantle only slightly closed; usually there is only a single suture.
Siphons absent or very short. Gills smooth. Nearly always dimyarian.
Shell equivalve, with an external ligament.
Fam. 1. _Dreissensiidae._--Shell elongated; hinge without teeth;
summits of valves with an internal septum. Siphons short.
_Dreissensia_; lives in fresh water, but originated from the Caspian
Sea; introduced into England about 1824.
Fam. 2. _Modiolarcidae._--Foot with a plantar surface; the two
branchial plates serve as incubatory pouches. _Modiolarca._
Fam. 3. _Astartidae._--Shell concentrically striated; foot elongate,
without byssus. _Astarte_; British. _Woodia. Opis_; Secondary.
_Prosocoelus_; Devonian.
Fam. 4. _Crassatellidae._--Shell thick, with concentric striae,
ligament external; foot short. _Crassatella. Cuna._
Fam. 5. _Carditidae._--Shell thick, with radiating costae; foot
carinated, often byssiferous. _Cardita. Thecalia. Milneria._
_Venericardia._
Fam. 6. _Condylocardiidae._--Like _Carditidae_, but with an external
ligament. _Condylocardia. Carditella. Carditopsis._
Fam. 7. _Cyprinidae._--Mantle open in front, with two pallial
sutures; external gill-plates smaller than the internal. _Cyprina_;
British. _Cypricardia. Pleurophorus_; Devonian to Trias.
_Anisocardia_; Jurassic to Tertiary. _Veniella_; Cretaceous to
Tertiary.
Fam. 8. _Isocardiidae._--Mantle largely closed, pedal orifice small;
gill-plates of equal size; shell globular, with prominent and coiled
umbones. _Isocardia_; British.
Fam. 9. _Callocardiidae._--Siphons present; external gill-plate
smaller than the internal; umbones not prominent. _Callocardia_;
abyssal.
Fam. 10. _Lucinidae._--Labial palps very small; gills without an
external plate. _Lucina_; British. _Montacuta_; British.
_Cryptodon._
Fam. 11. _Corbidae._--Shell thick, with denticulated borders; anal
aperture with valve but no siphon; foot elongated and pointed.
_Corbis. Gonodon_; Trias and Jurassic. _Mutiella_; Upper Cretaceous.
Fam. 12. _Ungulinidae._--Foot greatly elongated, vermiform, ending
in a glandular enlargement. _Ungulina. Diplodonta_; British.
_Axinus_; British.
Fam. 13. _Cyrenellidae._--Two elongated, united, non-retractile
siphons; freshwater. _Cyrenella. Joanisiella._
Fam. 14. _Tancrediidae._--Shell elongate, sub-triangular. Extinct.
_Tancredia_; Trias to Cretaceous. _Meekia_; Cretaceous.
Fam. 15. _Unicardiidae._--Shell sub-orbicular, nearly equilateral,
with concentric striae. Extinct, Carboniferous to Cretaceous.
_Unicardium. Scaldia. Pseudedmondia._
Fam. 16. _Leptonidae._--Shell thin; no siphons; foot long and
byssiferous; marine; hermaphrodite and incubatory. _Kellya_;
British. _Lepton_; commensal with the Crustacean _Gebia_; British.
_Erycina_; Tertiary. _Pythina. Scacchia. Sportella. Cyamium._
Fam. 17. _Galeommidae._--Mantle reflected over shell; shell thin,
gaping; adductors much reduced. _Galeomma_; British. _Scintilla.
Hindsiella. Ephippodonta_; commensal with shrimp _Axius_. The three
following genera with an internal shell probably belong to this
family:--_Chlamydoconcha_. _Scioberetia_; commensal with a
Spatangid. _Entovalva_; parasitic in _Synapta_.
Fam. 18. _Kellyellidae._--Shell ovoid; anal aperture with very short
siphon; foot elongated. _Kellyella. Turtonia_; British. _Allopagus_;
Eocene. _Lutetia_; Eocene.
[Illustration: FIG. 28.--Lateral view of a _Mactra_, the right valve
of the shell and right mantle-flap removed, and the siphons retracted.
(From Gegenbaur.)
br, br´, Outer and inner gill-plates.
t, Labial tentacle.
ta, tr, Upper and lower siphons.
ms, Siphonal muscle of the mantle-flap.
ma, Anterior adductor muscle.
mp, Posterior adductor muscle.
p, Foot.
c, Umbo.]
Fam. 19. _Cyrenidae._--Two siphons, more or less united, with
papillose orifices; pallial line with a sinus; freshwater. _Cyrena.
Corbicula. Batissa. Velorita. Galatea. Fischeria._
Fam. 20. _Cycladidae._--One siphon or two free siphons with simple
orifices; pallial line simple; hermaphrodite, embryos incubated in
external gill-plate; freshwater, _Cyclas_; British. _Pisidium_;
British.
Fam. 21. _Rangiidae._--Two short siphons, shell with prominent
umbones and internal ligament. _Rangia_; brackish water, Florida.
Fam. 22. _Cardiniidae._--Shell elongated, inequilateral. Extinct.
_Cardinia_; Trias and Jurassic. _Anthracosia_; Carboniferous and
Permian. _Anoplophora_; Trias. _Pachycardia_; Trias.
Fam. 23. _Megalodontidae._--Shell inequilateral, thick; posterior
adductor impression on a myophorous apophysis. Extinct. _Megalodon_;
Devonian to Jurassic. _Pachyrisma_; Trias and Jurassic. _Durga_;
Jurassic. _Dicerocardium_; Jurassic.
Fam. 24. _Unionidae._--Shell equilateral; mantle with a single
pallial suture and no siphons; freshwater; larva a glochidium.
_Unio_; British. _Anodonta_; British. _Pseudodon. Quadrula.
Arconaia. Monocondylea. Solenaia. Mycetopus._
Fam. 25. _Mutelidae._--Differs from _Unionidae_ in having two
pallial sutures; freshwater. _Muleta. Pliodon. Spatha. Iridina.
Hyria. Castalia. Aplodon. Plagiodon._
Fam. 26. _Aetheriidae._--Shell irregular, generally fixed in the
adult; foot absent; freshwater. _Aetheria. Mulleria. Bartlettia._