AUXILIARY ORGANS — OPERCULUM. 
125 
organic or chitinous substances, the shell having 98'94 per cent, of 
inorganic and 1 '05 per cent, of organic matters. 
These analyses of .shells and opercula made for me by IMr. Crowther, 
still further demonstrate that the composition of the calcareous parts 
of the shell and operculnm are not strictly identical in character, as 
the calcareous matter of the shells rapidly dissolved and disappeared 
upon immersion in h 3 'drochloric acid, while the calcic constituents of 
the opercula were very difficult to dissolve, even in warm acid. 
It has been suggested by Adanson, Gray, and others, that the shell 
and its operculnm were together homologous with and represented the 
two valves of the Bivalve, hut Huxley considered the operculum to 
be the analogue, if not the homologue, of the byssus of the Bivalve, 
and that it cannot represent the valve of the shell, which is a palllal 
structure ; there is, however, a somewhat striking analogy between 
them, as the operculum is attached to the shell itself tlirough the 
body of the animal by a powerful muscle corresponding to the 
posterior adductor of the Lamellibranchs, which it may be also con- 
sidered to represent in function. Another remarkable peculiarity 
possessed in common, is that when the month of the shell is closed 
by a spii’ally coiled operculum the spiral always revolves in the oppo- 
site direction to the spire of the shell, a dextral .shell having a 
sinistral operculum and vice versa, exactly as is the case with the two 
shells of a Bivalve, the right valve of which may be twisted or coiled 
at the umbo to the right, in which case the left valve would he coiled 
or turned to the left. In those species like Neritina fiivlatUis, the 
similarity is still further marked by the development of complex 
calcareous processes on the inner side of the operculum close tQ the 
columella, which have been designated as the “rib” and “peg,” the 
rib functioning similarly to the interlocking teeth in bivalves, and 
even in species with a lamellate or concentric operculum, the nucleus 
or representative of the umbo is always nearest to the columella, or 
what would be the hinge line. The different forms of opercula are 
characterized as 
Spiral when .spirally formed; the coiling is sinistral in dextral 
shells and dextral in those sinistrally coiled, and the increase 
is derived from a portion only of the operculigerous lobe, exactly 
as the increase in size of shells is due to the mantle margin. 
Tlie spirally coiled opercula actually revolve or turn upon the 
columellar muscle during the progue.ss of growth, as is slunvii 
