Chapter XXII: Part 22

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._

Sub-order VIII.--_Adesmacea._

Ligament wanting; shell gaping, with a styloid apophysis in the
umbonal cavities. Gills prolonged into the branchial siphon. Mantle
largely closed, siphons long, united. Foot short, truncated, discoid,
without byssus.

Fam. 1. _Pholadidae._--Shell containing all the organs; heart
traversed by the rectum; two aortae. Shell with a pallial sinus;
dorsal region protected by accessory plates. _Pholas_; British.
_Pholadidea_; British. _Jouannetia. Xylophaga_; British. _Martesia._

Fam. 2. _Teredinidae._--Shell globular, covering only a small
portion of the vermiform body; heart on ventral side of rectum; a
single aorta; siphons long, united and furnished with two posterior
calcareous "pallets." _Teredo_; British. _Xylotrya._

Sub-order IX.--_Anatinacea._

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._

Order IV. SEPTIBRANCHIA

Gills have lost their respiratory function, and are transformed into a muscular septum on each side between mantle and foot. All marine, live at considerable depths, and are carnivorous.

Fam. 1. _Poromyidae._--Siphons short and separate; branchial siphon
with a large valve; branchial septum bears two groups of orifices on
either side; hermaphrodite. _Poromya_; British. _Dermatomya.
Liopistha_; Cretaceous.

Fam. 2. _Cetoconchidae._--Branchial septum with three groups of
orifices on each side; siphons short, separate, branchial siphon with
a valve. _Cetoconcha (Silenia)._

Fam. 3. _Cuspidariidae._--Branchial septum with four or five pairs of
very narrow symmetrical orifices; siphons long, united, their
extremities surrounded by tentacles; sexes separate. _Cuspidaria_;
British.

AUTHORITIES.--T. Barrois, "Le Stylet crystallin des Lamellibranches,"
_Revue biol. Nord France_, i. (1890); Jameson, "On the Origin of
Pearls," _Proc. Zool. Soc._ (London, 1902); R. H. Peck, "The Minute
Structure of the Gills of Lamellibranch Mollusca," _Quart. Journ.
Micr. Sci._ xvii. (1877); W. G. Ridewood, "On the Structure of the
Gills of the Lamellibranchia," _Phil. Trans. B._ cxcv. (1903); K.
Mitsukuri, "On the Structure and Significance of some aberrant forms
of Lamellibranchiate Gills," _Quart. Journ. Micr. Sci._ xxi. (1881);
A. H. Cooke, "Molluscs," _Cambridge Natural History_, vol. iii.; Paul
Pelseneer, "Mollusca," _Treatise on Zoology_, edited by E. Ray
Lankester, pt. v. (E. R. L.; J. T. C.)