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.)
