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.)