PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
197 
of other mineral substances. In this they differ from 
such structures as the bones of vertebrates, the eggs of 
bird*, or the armour of the crab, in which phosphate of 
lime forms the chief mineral basis. The proportion of 
animal to mineral matter differs in different species, but in 
marine shells the former may be estimated roughly at 
about one-tenth, while in land shells it constitutes nearly 
one-fifth of the whole. I have already mentioned that the 
cells of the mantle separate the mineral matter out from 
the blood, but the question may be asked—how does it 
come to be in the blood in the first instance ; in other 
words, whence does the mollusk derive its supply of lime? 
I reply, from the plants on which it feeds, or, if it be an 
animal-feeder, from the plants on which these animals in 
turn have fed. For our present purpose this might be 
considered a sufficient reply, but I may be allowed a 
slight digression in order to trace the process from an 
earlier point—I do not say from its earliest point, for it 
presents us with one of those “ stories without an end ” of 
which the economy of Nature furnishes so many examples. 
Far back from the sea-shore, on the mountain's side, you 
may have watched the beginnings of some tiny stream, 
and have speculated on the vicissitudes it would have to 
undergo before it came to rest at last in the great ocean, 
but you may not have reflected that this was one of 
Nature’s countless laboratories, in which she was preparing 
the needful stores for the myriad forms of animal and 
plant life contained in that ocean. Yet so it is. The 
rain, as it descends through the atmosphere, and as it 
soaks through soil composed of decaying vegetable matter, 
drinks in a supply of carbonic acid, and, thus charged, 
percolates through the pores of the rocks, dissolving the 
lime from them as it goes. Therefore, when if comes to 
the surface again as the spring on the hill-side which forms 
the fountain-head of the mountain stream, it is the bearer 
of an invisible store of mineral wealth, and as it flows 
along, and joins with other streams to form the brook 
and then the river, it constantly dispenses this store of 
lime to plants and animals which either live in its waters 
or derive their supplies from it indirectly. When it comes 
at last to the sea, it delivers up this burden to the 
molluBks, the coral polyps, the microscopic foraminifera, 
and the hosts of other creatures that build up a limey 
framework. These creatures, receiving this invisible car¬ 
bonate of lime through the medium of the plants on which 
they feed, are able to convert it into the visible bi-carbonate 
of lime by the addition of a further atom of carbonic acid 
in the microscopic laboratories of their cells. On parts of 
the coast which are not fed by streams, and where the 
rocks of the shore themselves do not contain lime, we find 
that shells are either very scarce or absent altogether, and 
frequently those that are present are thin and fragile, 
owing to the scarcity of building material. For the same 
reason, land shells are always more abundant and better 
developed in districts where lime is plentiful either in the 
rocks or in the soil. I have more than once been aston¬ 
ished in exploring a Highland glen to come upon large 
numbers of snails in a comparatively small area, until I 
found that this area marked the outcrop of a bed of Silur¬ 
ian limestone. It seems to be doubtful whether mollueks 
derive their lime exclusively through the medium of food- 
plants, or whether they have to a certain extent the power 
of assimilating it direct from the water or from the rocks, 
but certainly by far the greater quantity is derived by the 
former means. 
Returning once more to the growth of the shell, you 
may ask—how are all the beautiful colours and markings 
and patterns to be accounted for ! These, too, are the 
work of the minute cells composing the fleshy edge of the 
mantle, by means of which the mollusk, by a marvellous 
provision of Nature, can separate out different colour pig¬ 
ments from its blood, can arrange them in a certain definite 
order, often of great complexity, and can add these to the 
walls of its dwelling during the building process. But 
to my mind far more marvellous is it that each 
species should decorate its shell, not at random, but, 
with slight variations, in the same way that its 
progenitors decorated theirs. The colouring matter 
is generally confined to the outer layer of the shell, 
but in some the deeper parts are coloured also. 
This is the case with the various species of Cassis, whose 
shells are composed of variously-tinted layers, ranging 
from white to pink, claret or chocolate, as the case may be. 
It is by carving these different layers that the Italian 
cameo-cutter can produce such exquisite effects. The in¬ 
tensity of the eolour of shells depends greatly on the 
amount of sunlight they receive. Hence the brilliant hues 
of tropical species, and hence also the comparatively 
colourless shells of mollusks which live at considerable 
depths of the ocean. 
Some interesting questions arise in regard to the relation 
of the forms of shells to their growth. As you are aware, 
almost all shells are either univalve or bivalve; that is, 
consist of one piece, or two pieces. The former, moreover, 
constituting about three-fourths of the whole molluscan 
sub-kingdom, are generally spiral in form. Now, if we 
suppose a spiral shell to be made plastic, and to be uncoiled, 
we shall have a tube tapering to a closed end, or, in other 
words, a cone, more or less drawn out. A cone, then, is said 
to be the type or plan upon which univalve shells in general 
