VARIATION IN COLOUR. 
S7 
Tliat tlii« iirisniatic colouring is largely due to the light rays iiu- 
piiiging on the iuuuinerahle tiiiely incised lines is demonstrated by the 
fact that the iridescent eftect has been successfully imitated on steel 
buttons by engraving u})on them similar fine and parallel lines. In 
some species this beautiful effect is probably to some extent due to 
the thin laminated structure of the naci’eous layer. All nacreous 
shells, however, when polished become iridescent and form more or 
less brilliant mother-of-pearl. 
The colouration of the body of the animal, speaking generally, 
would often appear to have little or no relation to that of the shell, 
for Helix nemondis, which has the most brilliantly and variously 
coloured shell of any of our native species, has an almost unifoimly 
coloured animal, and uniformly horn-coloured shells are not always 
borne, as might be e.xpected, upon uniformly coloured animals, Imt on 
the contrary are sometimes, as in Limua-a, tenanted by animals with 
the mantle strongly maculated and marbled with black and yellow. 
The naked forms are also more brilliantly and variously coloured than 
the animals of the testaceous species. 
Shells which live most freely e.xposed to the light, if other circum- 
stances be fiivourable, are the most brightly coloured, as Ileliv nemondlx 
and Helix hurtensix, which sometimes almost rival tropical species in 
the brilliancy and variety of their colours and banding, varying from 
almost pure white, through yellow, pink, and chestnut, to deep 
chocolate brown, while those species usually found secreted during the 
day under stones, logs, etc., like the Hijaliniw, or which habitually live 
on the ground or the trunks of trees, like the Bidiiniid, approximate 
to these objects in aspect, their colouring being Procryptic or pro- 
tective in character, the e.xceptions being the white or colourless 
species, which however are usually of small size and very retiring in 
habit. 
Shells like those of Limax, which are developed within as well as 
concealed by the mantle and thus e.xcluded from light, are always 
white or colourless, and do not exhibit the diverse and varied tinting- 
acquired by external shells. 
In clear bright water the Aiiodmis and Unionea are often of a 
bright green coloui’, sometimes beautifully rayed by a deeper tinge 
of the same hue or by distinctly yellow or brown shades of colour ; 
according to Herr .Jordan the radiate varieties are chiefly luit not 
invariably found in flowing waters ; but the shells inhabiting the 
