324 MOLLUSCA. 



Class I. 

 LAMELLIBRANCHIATA* (PELECYPODA. LIPOCEPHALA). 



Symmetrical Mollusca without head or odontophore; with bilobed 

 mantle, bivalve shell, and usually lamellate gills. 



The name of this class f is taken from the form which the gills 

 present in most members of the group. The fresh-water mussel 

 Anodonta cygnea is a typical member of the class. 



The mantle-fold is in reality a Continuous annular fold of the 

 dorsal integument, but in connection with the lateral compression 

 of the body it is especially developed on the two sides of the 

 animal into a right and left lobe. It is, therefore, usually described 

 as being bilobed. The mantle secretes over the whole of its outer 

 surface a cuticular covering, which becomes calcified everywhere 

 except in the dorsal middle line, forming the two pieces, or valves, 

 of which the shell is composed. In the dorsal middle line it 

 remains as an uncalcified elastic membrane the hinge-ligament 

 which connects together the two valves of the shell. The latter 

 are rarely exactly alike ; nevertheless, the term unequivalve is 

 only applied to those shells in which the asymmetry is very 

 marked and the valves can be distinguished as an upper and 

 lower. In such forms (Ostreidae) the lower valve is the larger 

 and more arched, while the upper is smaller and flatter, closing 

 up the concavity of the lower after the manner of an operculum. 

 The two valves of the shell are in contact dorsally, where they 

 are also connected by the ligament. This line of contact may be 

 complicated by the presence of interlocking hinge-teeth. The hinge- 

 edge with the ligament is, therefore, to be distinguished from the 

 free edge of the shell, which is divided into an anterior, ventral, 

 and posterior (siphonal) edge. The anterior and posterior edge 



* G. Cuvier, " L'histoire et V anatomic des Mollusques" Paris, 1817. Bojanus, 

 " Ueber die Athem und Kreislaufswerkzeuge der ZAveischaligen Muscheln," Isis, 

 1817, 1820, 1827. Deshayes, "Histoire Naturelle des Mollusques" (Exploration 

 . de I'Algtrie), 1844-1848. S. Loven, K. Vet. Akad. Hanlgr. Stockholm, 1848. 

 Translated in the Arch. f. Naturgesch., 1849. Lacaze Duthiers, Ann. des Sc. 

 Nat. 1854-1861. H. and A. Adams, " The Genera of recent Mollusca" London, 

 1853-1858. L. Reeve, " Conchologica iconica," London, 1846-1858. R. H. Peck 

 "The Minute Structure of the Gills of Lamellibranch Mollusca," Quart. J. Mic. 

 Sci., vol. xvii., 1877. K. Mitsukuri, "Structure and Significance of some aber- 

 rant forms of Lamellibranchiate Gill," Quart. J. Mic. Sci., xxi., 1881. P. 

 Pelseneer, "Contribution a 1' etude des Lamellibranches," Arch. d. Biologic, xi., 

 1891. P. Pelseneer, "Introduction a V etude des Mollusques," Bmxelles, 1894. 



t There does not appear to us to be any sufficient reason for altering this well- 

 established name in favour of the less suitable, but perhaps more symmetrical 

 term, Pelecypoda (TreXe/cus, an axe). 



