126 
AUXILIARY ORGANS — OPERCULUM. 
Fig. 27S. 
Ciiliim X 3. 
C}'ciosionin clc^aus (MiilL), 
l»ox Hill, Surrey, 
Collected Uy Mr. W.Whilwcll, F.L.S. 
liy tlie fact that the termination of the .siiiral ivliere the enlaroe- 
inent is exclusively made always ahnts against a certain definite 
part of the aperture of the shell, and thus clearly establishes that 
the oi)erculum must of necessity, in the course of growth, 
actually revolve upon the end of the columellar muscle as many 
times as there are whorls or coils indicated thei’eon. Tliese 
s])iral opercula may he distinguished as 
r.^ucLspiRAL, or few whorled, 
the nucleus being placed 
towards the basal or an- 
terior end, as in Ci/chatooxi 
c/cr/G/LS, Arinild I'mcdta ,Qt(\ 
'I'he o])orculum of Ci/rhi- 
is chiefly and 
very distinctly formed of two thin lamina', as its chan- 
nelled edge clearly indicates ; or 
Multispir.\l, or many whorled, the nucleus being situate 
towards the centre of the 
organ, as in ]’'((/raf(t 
iitills, hut the number of 
whorls iu tlie operculum 
has no nocessary relation 
to the number in the shell, 
tlnnigh at times iu accord 
with it. Dr. Jeffreys de- 
scribes and figures the o])erculuni of Vdh'atd jusrliiiih'a 
as formed of f(t to f 2 s]ural whorls, and Mo( pi in-Tan don 
figures the same as containiug only t.l- volutions, though 
descrihing it correctly in the text. 
Concentric when the increase in size 
tabes place from the whole surface 
of the lobe, ami more or less eipially 
aronnd the suh-central nucleus ; 
such opercula do not alter their 
])o.sition u])on the retractor muscle 
to which they are atli.xed, hut ac- 
commoilate themselves by simi>le 
gTowth to tit the iucreaseil size <jf the aperture of the shell, as 
ill Vminmi contectd, etc. 
Fir.. 270. — A MuUispirally-coilecl 
Operculum X 8. 
l’n/7'atn piscinafis (Mull.), 
River 'rarff, Kirkcutlbriglu, 
Collecteil by Mr. F. K. Coles. 
Fig. 277. — A Concentric Oper- 
culum X 11. 
i'k>ipara conh'cta Millet, 
Southport, 
Collected by Mr. CJ. \V. Chaster. 
