VARIATION IN ARMATURE OF APERTURE. 
69 
FiTt. 151. — Lbnna'a stag7iaUs var. Fig. 152. — Helix 7te7no7‘aUs L., 
variegata Ha/ay, Bitton, near Bath, 
Peffer Burn, Haddingtonshire, Collected by Miss F. M.'Hele, 
Coll, by Rev. Dr. McMurtrie, F. R.S.E. , 
Showing in a fluviatile and in a terrestrial species the transverse 
thickenings, marking rest periods and indicating the growth checks 
sustained by the animal. 
and allowing only the merest film of shell matter to be deposited 
there, as they fit into and form the interstices between the apertural 
ridges moulded by the spaces of the mantle folds. Those species with 
the retractor muscle affixed high up the spire and which tlierefore 
withdraw far within the shell, form the strongest apertural plaits 
with the greatest prolongations into the interior of the shell. 
The normal thickening of the shell towards the aperture, which in 
some species culminates in the form of a raised internal ridge or rib 
parallel to and 
within the outer 
or palatal mar- 
gin of tlie shell, 
is doubtless a 
response to the 
necessity for in- 
creased strength 
atthat partinthe 
mature animal. 
In the immature 
stages of a mollusk, analogous transverse thickenings may occasionally 
occur at regular or irregular intervals during growth, and some 
species, as Limncca glahra and Limnwa stagnalis amongst aquatic 
and Helix arhustoruin and Helix nemoralis amongst terrestrial 
species, are especially addicted to this 
habit, which is assumed to be an 
outward indication of growth checks 
sustained by the animal, owing to some 
temporarily unfavourable conditions 
of the environment. 
In some of the aquatic species, 
like Planorbis spirorhis, the inhibi- 
tion of growth owing to exposure to 
drought is also stated to determine the formation of this abnonnal 
rib or thickening near the apertural margin, this peculiarity being- 
shared by some of the Limnwcv ; a plenitude of calcareous matter 
will naturally fircilitate the formation of this sub-marginal internal 
rib, which is usually whitish or yellowish in tint, but in Ltmtuva 
pdlustris and Limnwa, stagnalis frequently assumes a deep purple 
or violet colour. 
Fig. 153. — Lii/mcea peregi-a var. 
jnajgiiiata Michaud X Iw, 
A pond, Lewes, Sussex, 
Collected by JMr. C. H. Morris, 
Showing the internal sub-marginal rib, 
indicative of exposure to drought or of 
a plethora of calcic matter. 
