3 22 
MOLLUSCS. 
Dentalium, or a Carinaria and a Turritella, and among the bivalves between a 
Pholas and the hammer-oyster (Malleus). The difference in size is still more 
remarkable, some microscopic forms weighing not more than the fiftieth part of 
a grain, whereas the gigantic Tridacna occasionally turns the scale at over 
500 lbs. 
As every mollusc has a history of its own,—a certain course to pursue in the 
living world,—its means of existence and propagation will be found sufficient, 
although to the human eye the chances against it may superficially seem over¬ 
whelming. The glassy Carinaria, regarded as a dweller on the surface of the 
ocean, would seem ill-adapted to withstand the buffeting of storms at sea; and so 
it is, but there is very little doubt instinct has taught the mollusc the proper 
moment, and the sufficient depth to descend from the surface, to be safe from the 
tempest’s violence. The dweller on the seashore, open to the onslaught of the same 
unpitying foe, defends himself with a stronger abode. What dash of the waves upon 
the undefended rock could in the slightest affect the conical shell of the limpet ? 
In very deep water the tendency to tenuity in the molluscan shell is plainly 
apparent, yet there are species dwelling at depths beyond the influence of surface 
storms, the shells of which present considerable strength and solidity. We may 
ask how and why is this ? Although we may not in all cases be able to answer these 
queries from actual knowledge, certain is it, that reasons, probably very near the 
truth, are easy of suggestion. For example, the large Cyprina islandica and the 
heart-cockle ( Isocardia cor ) of the British seas possess shells of considerable 
strength and solidity. These, being molluscs of large size, would prove dainty 
morsels to the hungry haddock or other fish, if they were unprotected by a power¬ 
ful shell and their extermination thus prevented. 
The varied colours of shells are due to glands situated on the 
Coloration. . ° 
margin of the mantle. In most cases the colour markings are placed 
on the outer surface of the shell, beneath the periostracum, but occasionally the 
inner layer of porcellaneous shells is of a different colour to the outer. This is well 
instanced in the helmet-shells (Cassis), which are employed by the carvers of shell- 
cameos to produce white or rose-tinted sculptures upon a dark ground. The colour 
in some shells is liable to extreme variation. Take, for example, the common 
hedge-row snails, Helix nemoralis and H. hortensis. Here we find the ground- 
colour yellow, brown, pink, white, lilac, and various intermediate shades, and the 
bands which are usually brown, and normally five in number, may be altogether 
absent or vary from one to six, their position also being equally variable. This 
diversity in colour-markings results from the different position of the pigment- 
glands upon the mantle margin, but the cause of this variation in the position has 
not been ascertained, although it may be presumed ; nor is the reason known of 
the difference of the ground-colour, which may occur among specimens of the same 
colony. White, black, red, green, yellow, olive, purple, slate-blue, and brown form 
the common ground-tints of shells, but pure blue is a colour hardly ever met with 
in the shells of molluscs. One or two species of land-shells (Corasia) from the 
Philippine Islands more nearly approach this tint than any other molluscs, but 
even in these there is a slight admixture of green. On the contrary, blue is a 
colour more commonly seen in the soft parts. The colour of the shell does 
