Completely spherical pearls can be formed only loose in the mantle or soft parts of the body of the animal; but intrusions incite a deposit of nacre in the form of a projection on the interior, which projection, often a mere bubble, but sometimes semi-detached, may take the shape and dimensions of the foreign substance. Or an inoffensive mollusc may be goaded by the piercing of its shell from the exterior to create that for which men venture into the depths of the sea. If a pearl-secreting oyster be inherently robust, its defence against assault from without may consist of the strengthening of the interior at the point of attack by deposits of nacre. Thus, a slight protuberance arises which becomes the base of a blister or button or the starting-point of a pear-shaped gem. Many a lovely gem is, therefore, nothing more than the imperishable record of aggression on the part of a flabby sponge on a resourceful oyster. Occasionally valuable pearls are found within huge blisters. Such pearls originate, no doubt, in the ordinary way, but, becoming an intolerable nuisance on account of increasing size, are confined in nacre. One of the accompanying illustrations shows the fate which befell an infant chiton upon intrusion on a small black-lip oyster, and coincidentally the origin of a blister. The chiton family being notorious for stolidity, the infant could not have realised the risks of its trespass until the strait-jacket made its retirement impossible. The nacre has reproduced the details of the chiton's exterior with the fidelity of a casting, and further reveals the fact that it was alive when entombed, for its struggles to escape are solidified. This deliberate act of the oyster may not stand comparison with the stone of Pyrrhus's ring, which had the figure of Apollo and the nine Muses in the veins of it produced by the spontaneous handiwork of Nature without any help from art. The marvellous stone belonged to the fabulous past; the imprisoned chiton to the prosaic present. Another illustration is that of an accumulation of nacre which has assumed accidental resemblance to a miniature shark. It was found in a gold-lip pearl shell in Torres Straits. The like quantity in globular shape would represent a pearl of great value. A PEARL IN THE MAKING. On a calm and luminous day I waded, disrobed, in shallow water as limpid as the fictitious stream which legend says King Solomon improvised at the foot of his throne when the Queen of Sheba attended his court. Lifting her robes--for she imagined the crossing of the water to be a ceremonial device--the gorgeous Queen displayed her shapely calves. The water resting on the verge of the lovely Isle was as delusively clear, but was not deceptive.