70 
VARIATION IN SCULPTURE. 
Ill some of the Ileltces, as Ilelir hurtcm^'is and many other species, 
the regularly formed internal rib, which forms so conspicuous a feature 
of the oral aperture in the adult, is 
developed, according to Longe and 
i\ler, only at the maturity of the shell, 
owing to the atrophy of certain 
glands of the margin of the collar, 
which occupy a furrow or groove 
parallel with the margin and within 
which the projecting rib is moulded. 
The Sculpture in Univalves and Bivalves is most usually trans- 
verse or parallel with the lines of growth and the margin of the 
aperture. It is at times very strongly developed and has perhaps 
its most remarkable e.vample in Ih'lir 
(tcnli'dfd, where the lamelho are produced 
into a beautiful coronet of spines, a form of 
ornamentation which is unhpie among 
British species, but is more numerous 
amongst tropical genera. The incremental 
transverse striie are sometiniesconsiiicuously 
developed in forms that arc generally smooth or have scarcely dis- 
cernible growth lines, and occasionally specimens are met with in 
which there is an apparent rhythm in the alternate series of tine and 
coarse incremental lines, suggestive of positive transverse sculpture, 
which may be sujiposed to be due to the more vigorous growth at 
s variation in the prominence of the 
growth lines is sometimes very regu- 
lar in its recurrence and produces 
the ribbed aiipearance seen in Ildix 
pnlchcUd var. codata, Ilelix arhiis- 
toriuti var. rudd, etc. Reeve has 
stated that those mollusks developing 
superficial sculpture are invariably 
smaller than individuals of the same 
species in which any kind of decora- 
tive sculpture is avoided. This ob.served fact will account for the 
di.sparity in size of Planorlih ]amt'dcm and its variety crista, a 
characteristic so constant as to have led to the variety being con- 
one time than another. T1 
I k;. \ I cli.x arbitsicrrum var. rudis 
Mulilf. (^lij'hlly enlarged), 
Craig Farm, Monlro.se, 
Collected by Mr. Win. Duncan, 
Showing the regular development t)f 
rihlets on an ordinarily .somewhat smooth 
.shell. 
Fig. 155. — //. aculcata Mull. X8, 
I’as.senthwaite, Ciunljerland, 
Collected by Capt. W. J. Farrer, 
Showing the corona of .spines. 
Fig. 151. — Helix horiensis Muller 
(slightly enlarged), 
Fordingbridge, Hants,, 
Collected by I^Ir. H. Richardson, M.A., 
Showing the internal sub-marginal rib 
indicative of maturity. 
