VARIATION IN COLOUR. 
8 (; 
Oligochromic, for the more soberly coloured and less varied shells 
of the temperate latitudes. E-xamples : Helix pisana Muller, 
iMeutone, pi. ii., tig. 10, and Helix nemomlis var. UbelUda 
(llisso), Bath, pi. ii., tig. 11; and 
Monochromic, for the simply coloured species characterizing the 
colder regions. Examples: Vitrina pelliicida^&v. depressiuscida 
Jetfr., Exeter, pi. ii., fig. 12, and Verticio (dpestris Alder, 
Cottingley, pi. ii., fig. 13. 
iMany exceptions naturally occur to these broad lines of distinction, 
as these results are naturally more or less influenced and modified by 
nature of soil, vegetation, altitude, and other circumstances, but 
there can be no dispute that they express the leading features of 
colour distribution upon the globe, though species with strong shells 
are usually more vividly coloured than their congeners possessing 
fragile and delicate ones. 
Colouring in the mollusca may he due to the structural character 
of the surface of the shell diffracting and dispersing the light rays, 
but is more generally owing to the ahsori)tiou of certain of the 
elements of light by the s})ecial substances in the shell termed pig- 
ments, the i)articular colours produced varying according to the 
chemical nature of the cou.stituents of the shell disi)laying the colour, 
and being due to those rays or vibrations of light which are not 
ahs(jrhed, hut reflected to the eye ; thus red is the result of the 
absorption of all hut the red rays which are therefore reflected and 
visible, while black is owing to the absori)tion of all the constituents 
of light, and white to their complete reflection. 
Structural (jr iridescent colouring is strikingly exhibited by the 
inside of the valves of the Naiads, as well as occasionally by the 
interior surface of some Univalves, and is 
owing not to the varied absorptive power 
of certain of the component parts of the 
shell, but to a multitudinous series of 
undulating parallel lines of exceeding 
fineness which diffract the light and give 
rise to a i)rismatic play of colour, varying 
from red, through yellow, gveeu and blue 
to violet, according as the point of view 
becomes more and more obli(pie and the 
lineation thereby more closely approximated, owing to the varying 
angles at which the surface is seen. 
Fig. 192. — Highly magnified sur- 
face of a nacreous shell, showing the 
minute parallel sculpture upon which 
structural or prismatic colouring is 
dependent (after Tryon). 
