AUXILIARY ORGANS — CLAUSIUM. 
127 
Fig. 280. Fig. 281. 
An Articulated and Sub-spiral Operculum X 4. 
Ncritina /lu7naiitis (L.), 
Canal, Northampton, 
Collected by Mr. W. I). Crick, F.G.S., 
Fig. 280 showing the external face and the subspiral 
formation of the operculum, with the projecting 
“rib,” and Fig. 281 showing the interior face with 
sub-marginal muscular scar, the “rib” and “peg," 
and anterior scar, at base of the apophyses. 
r. rib or fulcrum ; /. rudimentary peg. 
Articulated when furnished with more or less complex projections 
or apophyses, which 
appear to act like a 
hinge, as in Nerit'ma 
fluviatilis. The oper- 
cnlnm of this species 
has a broad flexible 
margin, and is duplex 
in character, being dis- 
tinctly and marginally 
snbspiral on the ex- 
ternal face, but this feature is practically obscured on the 
internal face by a later calcic deposit. 
The different parts of the operculum may he distinguished accord- 
ing to its position when applied to the month of the sliell, the Exterior 
face lieing that facing outw'ardly and the Internal face that to which 
the columellar muscle is attached; the part adjoining the base of the 
aperture of the shell may likewise be termed the base or Anterior end 
of the operculum, the opposite end being the upper or Posterior one, 
the Palatal margin of the operculum is the one adjacent to tlie palatal 
or outer lip of the shell, and the Columellar side the one adjoining 
the columellar margin. 
The Clausilium, or Clausium (claimim, an enclosure), as I prefer 
to call it, is another ingenious contri- 
vance, formed within the last whorls of 
the shell in the genus Clam/l/a, also for 
the purpose of closing the aperture 
against outside enemies and preventing 
desiccation. It is an elongate, some- 
what oval, externally convex, and more 
or less arcuate, nacreons-white plate or lamella, attaclied by a 
somewhat cartilaginous, elastic and - 
spirally twisted pedicle or footstalk to 
the columella near the commencement 
of the penultimate whorl, and has been 
aptly compared to a door furnished 
with an elastic spring. It also varies 
greatly in its form and character in the 
dift’erent species and in coiTelation with 
"W 
Fig. 282. —Clausium of Clausilia 
Oidentata Strom X 8, 
Stroud, Gloucestershire, 
Collected by Mr. E. J. Elliott, 
Showing the convex external face 
of the clausium and its elastic pedicle 
of attachment. 
Fig. 28.3. — Clausium of Clansilia 
cravcnensis Taylor x 8, 
Starbolton, Yorks., 
Collected by Mr. W. I.). Roebuck, 
Showing the convex external face 
of the clausium anrl its elastic pedicle, 
for comparison with that of Clausilia 
bhicntata (hg. 282). 
