4. JONES : MOLLUSCAN ALBINISM, 
_ With this brief glance at albinism generally, let us turn to 
albinism from a strictly conchological point of view, and in 
doing so, a few words as to the structure and colouring of the 
shell, and the various parts of the animal from which they are 
produced, may not be out of place. 
The shell consists of three layers. The most external, the 
epidermis or ectoderm, is chitinous ; the middle, the thickest, 
is calcareous, and sometimes called the prismatic ; the third, or 
nacreous, also calcareous, is always smooth and shining and 
frequently beautifully coloured. The external and middle 
layers are formed in the Gastropoda by the collar, and in the 
Pelecypoda by the free edge of the mantle, which corre- 
sponds to the collar of the former class. 
In both orders the nacreous layer is secreted by the mantle. 
The colouring of the shell. There are two kinds of colour- 
ing: that produced by the deposition of particles in such a 
manner as to break up the light and give rise to the idea of 
colour; and that due to the deposition of particles of colouring 
matter, which matter is known as melanin. ‘The shell of an 
Anodon gives a good illustration of both varieties, the pearly 
inside being a good sample of the first, and the green ectoderm 
of the second. ‘The ectoderm is generally coloured, the nacre- 
ous layer less frequently, and the prismatic but rarely. The 
ectoderm is developed from the collar, therefore the collar plays 
the most important part in the colouring of the shell as a whole. 
Indeed, the collar presents a highly complex organ, producing 
as it does two different kinds of shell material and the colouring 
material of the ectoderm at one and the same time. There are 
many minor points, very interesting in themselves, with regard 
to the colouring matter of the shell; but, as they do not bear 
directly on the subject in hand, and would take up much space, 
I am forced to leave them untouched in this paper. 
A mollusc bearing an albino shell presents a paradox. The 
shell may be perfectly devoid of colour, but the animal inhabit- 
ing it is always more or less pigmented. In short, a shell-bearing 
J.C., viii., Jan. 1895. 
