VARIATION IN BANDING. 
95 
The yellow or xanthous variety, Uhellula, of Helix nemoralls is often 
noticed to pi’esent a strong rosy tint upon the earlier or apical whorls, 
and this peculiarity is occasionally found so distinctly marked that a 
special varietal name has been used to indicate it. 
The Helix picta figured (pi. ii., fig. 9) also shows the intimate 
relationship existing between erythrous and xanthous colouring, as 
both colours are blended together upon the penultimate and preceding- 
whorls. Indeed the shell, like other organs of the body, undergoes 
in some species great mutations, in colour, as well as in markings 
and other characters in the progress of its growth to maturity. 
The real colour of the shell is sometimes greatly modified to the 
eye by the colour of the chitinous periostracum, which may vaiy 
from the pure white of the albine form, through the delicate i)rimrose 
tint of the xanthochroic varieties, as seen in Helix arhustorum and 
Helix aspersa (see pi. i., fig. 1), to the deep dark brown, as found 
in Helix aculeata and other species, and according to its depth of tint 
and thickness obscures and modifies the colour of the calcareous 
surfirce beneath ; thus some of the yellowish brown and other varieties 
of Helix iiemoraUs, etc., when denuded of their epidermis are found to 
be of a more or less intense and livid violet colour, but other 
Univalve species may chiefly or entirely owe their colouring to the 
chitinous covering of the shell. 
The Banding, or coloured markings, in our British Univalves have 
usually a spiral character, the corresponding markings in the Bivalves 
being those radiating from the umbones to the free margins of the 
shell, thus differing diametrically from the general direction of the 
sculpture, which is usually transverse or coincident with the lines of 
accretion. These varied markings are formed by the glands of the 
mantle margin, and their continuous exercise during growth produces 
a connected and necessarily spiral band in the Gastropoda, and the 
radiate markings in the Pelecypoda, their intermittent action giving 
rise to transverse markings, spots, or blotches according to the ex- 
tent of the development of the colour glands and the greater or lesser 
intervals taking place between the periods of their secretory activity. 
Mr. W. II. Dali has remarked that the tendency to striped markings 
wonld probably aid in the concealment of the shells amongst the lights 
and shadows of the grass and herbage, leading one to attribute these 
markings to similar causes to those that may have also led to the 
development of the striped markings of the Tiger. 
