1 24 AUXILIARY ORGANS — OPERCULUM. 
'I'lie position of the operculum ui)oii the aniiiuil varies within cer- 
tain limits in the ditl’erent species. In Xeritiua, when the animal is 
crawling the opercnlum practically hinges 
on the columella, hut in other genera it 
may be at a comparatively considerable 
distance from the aperture. The oi)er- 
culum is brought over the mouth of the 
shell to close the aperture when the 
animal retires, by the contraction of the 
retractor muscle to which it is attached ; the point of insertion usually 
leaves a single scar upon the opercnlum, hut on that of Nerithui 
j! iiciutiHs{\\Q\-Q aretwo scars, that formed by the hinder muscle being the 
largest and forming a broad snhmarginal scar along the side adjacent to 
the columella, while the scar at the anterior end is .smaller and is of a 
somewhat ovate shape and situate at the ha.se of the opercular proce.ss, 
termed the “p^'S. ” "hicli is, however, rudimentary in our Briti.sh 
sjiecies, ami therefore usually (|uite (»verlooked by imjst conchologi.sts. 
'I'lie muscles are attached to the shell at opposite ends of the pillar lip. 
'I'lic operculnm varies even more than the shell in the relative 
amounts of chitinous and calcareous matter contained in its composi- 
tion. In some sjiecies, as Xeritiua j/iu'i(cfi/ir, the operculum has 
comparatively less organic and more calcareous suhstances than the 
.shell who.se mouth it closes, while that of \"irip(ira c/ivyiu/vt is purely 
chitinous, the gravimetric te.sts made upon clean oiiercula by Mr. 
Crowther showing no trace of lime, although the shell of that .species 
contained 11 1 '2 I jier cent of earthy .salts. 
'I’he compo.sitioii of the operculum of Xeritiua jlariatilis has been 
termed “not .shelly ” by some authors, and imrely calcareous by others, 
hut a careful analy.sis showed it to contain t)7';>2 percent of inorganic 
matter and 2 (;7 jau' cent, of (organic .substances ; the .shell of the same 
Species h(jwever contains more organic matter than its oj)erculum 
having 4'4() per cent, of organic substances and only Pd'ot) i)er cent. 
Ilf inorganic matter in its comj)osition. 'I’he organic skeleton of the 
operculum of this .species is not destroyed even by mixed acids, the 
chitiiunis framework preserving its form and elasticity uninii)aired 
and making a jnetty object under a lens. 
Ci/clartonia e/epaits, on the contrary, has le.ss chitinous matter in the 
shell than in its operculum, the analy.sis showing the operculum to 
contain ‘J2'7G iter cent, of inorganic matters and 7 "23 per cent, of 
Fig. 270. — Section througli the 
sliell o( //7c:uat///s, showing 
the operculum /« ivV/J and the mode 
in which the “rih" hinges upon the 
columella, X 3 (section cut by Mr. 
1'. Rhodes). 
