THE CHINESE PEARL-MUSSEL. 351 



substances like the common Mussel, but that in process of time it begins to bore its way into 

 the object to which it is moored. As the shell increases in size, the chamber is enlarged in 

 dimensions ; but the original aperture remains of the same diameter as when first bored, and 

 therefore effectually prevents the animal from making its exit. 



Some persons have suggested that the animal employs an acid for the purpose of dissolv- 

 ing the rocks ; but if such were the mode of operation, the shell would suffer equally with the 

 stone. A continual current of water forms the basis of another theory ; and provided that the 

 animal were sufficiently long-lived, there is no doubt but that the constant action of water 

 would in process of time wear away the stone, however hard it might be. But as yet no theory 

 has sufficiently accounted for the. fact that the creature excavates these chambers with won- 

 derful rapidity, and that, in all cases, the chamber corresponds with the shape and size of the 

 shell. It is evident, also, that the shell itself is not the means by which the chamber is bored, 

 as the peculiar shape of the hole prevents the shell from rotating. 



The Lithodomus seems to drive its curious tunnels through everything that comes in its 

 way, for, in one case, a specimen has bored through the upper part of the limpet-shell, broken 

 into a chamber already excavated by another individual, and forced its way fairly through the 

 inhabitant as well as the habitation. 



The animal is slightly luminous, as is the case with most of the burrowing mollusks. 

 The color of the shell is uniformly pale brown. 



The FINGER DATE-SHELL is a rock burrower, and so beautifully decorated that it seems a 

 sad pity to bury so lovely a shell in so dark a recess. At a little distance it is quite ordinary 

 in appearance, being apparently a plain, mahogany-colored shell ; but when examined closely, 

 it is found to be elegantly formed, colored with a peculiarly rich ruddy brown hue, and 

 sculptured with myriads of minute waved ridges and channels drawn crosswise over the shell, 

 which give wonderful effects of light and shade, and heighten the tints materially. The 

 animal is edible, and is eaten like that of the common Mussel. 



A RATHER curious-looking-shell, which, from its rude resemblance to the familiar toy 

 of childhood, is called the NOAH'S ARK. 



The Ark-shells are found all over the world, hidden under stones, in the crevices of rocks, 

 or even within the forsaken burrows of the pholas or the date-shell. Owing to their retiring 

 habits, and the nature of the localities in which they live, they are mostly distorted or 

 damaged. They can move themselves very fairly by means of a curious conical byssus, com- 

 posed of a series of thin plates, which can be cast off or re-formed at the will of the animal. 



EXAMPLES of pearl-bearing mollusks which inhabit the fresh waters, are the European 

 and the Chinese Pearl-mussel. 



The EUROPEAN PEARL-MUSSEL was once a valuable inhabitant of English rivers, on account 

 of its contents. It is now, however, seldom sought except for bait, and in the latter capacity is 

 more useful than in the former, as it is estimated that not more than one per cent, contain any 

 pearls, and not more than one per cent, of the pearls is of any commercial value. The older and 

 more irregular the shell, the better chance is there of finding a pearl ; and a diligent collector 

 may soon obtain a tolerable series of these objects for his. cabinet. Now and then, however, 

 a really fine pearl is found ; and one, that was obtained from the Conway, now holds a place in 

 the crown of England. This Mussel is tolerably active, and, if laid on the sandy or muddy 

 floor of an aquarium, will soon assume its usual attitude. 



The genus to which the CHINESE PEARL-MUSSEL belongs is distinguished by the thin 

 elastic wings into which the valves are produced. 



From this species the Chinese, those incorrigible tricksters, are in the habit of producing 

 imitation-pearls by a very simple process. A string of small shot is introduced between the 

 valves, and the animal restored to its native element. The irritation caused by the presence of 

 the foreign body forces the mollusk to deposit the nacreous secretion upon the intruding 

 substances, and after a while the shot are covered with layer upon layer of pearly substance, 

 the thickness of the coating depending upon the length of time occupied in the construction. 



