THE UMBILICUS AND COLUMELLA IN UNIVALVES. 
29 
coiling ; when fairly developed so that one or more of the previous 
whorls are visible, as in Ilellv the shell is said to be Umbili- 
CATEi) ; if the orifice is small, as in Helix gninnldta, it is said to lie 
Fig. 51. — A Widely Umliilicatcd Univalve. 
llcU.v itala Linne, 
Royston, Canibs., 
Collected by Mr. H. (b Fordham. 
Fig. o2. — \ Perforate Univalve. 
Helix _e^rnnulata .Alder X 2, 
Moslyn, Flint, 
Collected by Mr. W. 1). Roebuck, F.L.S. 
Pervious or Perforate ; when very compressed, or a mere fissure, 
it is termed Rimate, as in Viripara viripara (L.) ; but when the 
Fig. 53. — A Rim.ate Univalve. 
I’k'ipara Tivipara I.., 
Canal, Marple, 
Collected by Mr. William Moss. 
Fig. 54. — An Imperforate Univalve. 
Helix hortensis Midler, 
Fordinj?bridge, Hants., 
Collected by Air. H. Richardson, M.A. 
colnmella is solid the shell is then called Imuerforate ; some 
species such as Helix hnytennix, Helix dxperxa, etc., are umbilicate 
or perforate when young, but at maturity become imperforate by 
the e.xtension of the lip over the umbilical cavity (fig. 56). 
The Columella, or Pillar is the central portion or axis, real or 
imaginary, around which the whorls of the shell 
are coiled, but in very openly coiled or scalarid 
specimeii.s the columella is only theoretically exist- 
ent (p. 26, fig. 35). In the elongately spiral shells 
the columella, though 
always sinuous, may be 
nearly straight as in 
CUmsilid, but in other 
cases, as in many Helicex, 
it may be very tortuous 
and strongly twisted. 
Some species, as Neritina 
fuvidtilix and Cdryeliium 
niininuDu, absorb either wholly or in part not only 
Fig. 55. — Section 
through the shell of 
Clansilia Inininata 
(Mont). X 2, showing 
the nearly straight 
columella (from a sec- 
tion cut by Mr. F. 
Rhodes). 
Fig. 56. — Section through shell 
of Helix nenioialis L., showing 
the somewhat twisted and hollow 
columella and the method of closure 
of the umbilicus at the maturity of 
the shell (from a section cut by 
Mr. F. Rhodes). 
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