VARIATION IN CULOl'K. 
S8 
iiioic turbid rivers and ponds, or any waters eoiitaminated with 
sewage, etc., do not normally exldliit this bright and variegated 
colouring, but are often of various shades of brown verging 
occasionally upon black. 
In our Gastropoda the colouring matter is generally speaking 
resident in the calcareous part of the shell, and in the Pelecypoda 
is found in the periostracuin or epidermis, but the colouring of some 
Gastro])ods, as HeliJ' (icuhaUi, Chtus'dhi htmumtd, etc., is entirely or 
chietly due to the periostracal investment, and not to the pigmenta- 
tion of the calcareous part of the shell, such species resembling the 
Pelecypoda in this re.spect. According to Krukenberg the red, 
yellow and brown pigments, which are the most prevalent colours 
amongst mollusks, belong to the Lipochromoids or Melanoids. 
Prown of various shades is the most prevalent colour amongst both 
our land and freshwater shells, although some freshwater species are 
of a bright green, a colour not found among.st our land shells, the 
nearest approach to it being by Ihll.r recehitu, ivhich in some 
localities shows a strong though dull greenish shade. Camerano, who 
has studied the Mollusca generally with the object of determining 
the relative fre(piency of the various colours, finds that black is rare, 
brown, grey, yellow, white and red all common, violet relatively com- 
mon, while blue and green are more rarely met with. 
The i)inks and violets are usually the most evane.scent colour.s, and 
colouring generally amongst our land and fre.shwater mollmsks seems 
to be more or less fugitive, if abnormal or unusual as to position, 
thus the delicate purple .sometimes found on the lip of llcllx dxjierxd 
soon fades and disappears. The normal dark spiral banding of the 
fusco-labiate variety of Ihdlv hortenxh seems lU’actically permanent, 
hut the abnormal colouring of the li[) is unstable, the pink element 
first disappearing, followed more slowly by the brown. 
Laud .shells may be modified or changed in colouring by adventi- 
tious matter about their food, as Herr Ti.'^chbein has observed in 
Gei'inany that IIl-Hc liorteiit<U when found in the neighbourhood of 
tanneries a.ssumes a peculiar brownish hue. A(piatic shells are also 
liable to be similarly affected by substances iutermi.xed with or dis- 
solved in the water they inhabit, as Mr. Quilter has recorded that 
specimens of Unto pictorum found in a certain part of Groby Pool, 
Leicestershire, adjacent to a local out-crop of red triassic marl, had 
the nacre tinged of a beautiful salmon colour by the oxide of iron 
