426 THE FORK-TAILED DATE-SHELL. 



the constitution of the partaker, some being able to eat it with impunity, while others 

 who have shared the same meal are visited with asthma, violent rash, nausea, and many 

 other symptoms which, though not absolutely dangerous, are peculiarly annoying. The 

 Mussel is largely used for bait as well as for human consumption, more than thirty 

 millions being collected annually in one locality for that purpose. Little ill-shapen and 

 badly colored pearls are often found in this mollusc, but are quite useless for the 

 market. Attempts have been successfully made to propagate the breed of Mussels ; and 

 the vast plantations, as they may be called, of these creatures have increased to such 

 an extent, that they threaten to obliterate several useful bays for all maritime purposes. 

 The specimen which is represented in the engraving was found by Mr. Sowerby. 

 clinging to a wooden pile, at Harwich, some eighteen inches above the water, and hav- 

 ing six or seven pieces of stone, bits of shell, and similar substances, attached to the 

 byssus, among which it lay ensconced as is shown in the illustration. 



AN allied species, the DREISSENA, inhabiting the fresh waters, has of late years 

 rapidly overrun our island, having been originally imported into the Surrey Docks, 

 whence it has spread with astonishing fertility, passing from one river to another, 

 getting into all the little rivulets that trickle between meadows, and even obtaining 

 entrance into artificial basins by means of the water that feeds them through pipes. 

 About Oxford its spread was most notable, the many waters with which that city is 

 surrounded literally teeming with the mollusc very soon after its advent was first dis- 

 covered. The shell is like that of the edible Mussel, but shorter, and without the 

 beautiful nacreous lining. 



THE reader may observe in the upper portion of the engraving a number of little 

 shells buried deeply in some substance. These are specimens of the FORK-TAILED 

 DATE-SHELL, a little ochre-colored shell, without any peculiar beauty of form or color, 

 but yet as remarkable a creature as any that has been or will be mentioned. 



This little being has the power of burrowing deeply into the hardest stone ; and in the 

 present instance, the substance in which the Lithodomi are imbedded is a shell of the 

 gigantic limpet from Madagascar, measuring about six inches in diameter and half an 

 inch or so in thickness. The specimen, which I have carefully examined, is a really 

 wonderful one, the thick, hard, and solid substance of the shell being literally riddled 

 with the holes of the Lithodomus, whose forked processes just project from the circu- 

 lar aperture much like the eggs of the common scatophagus from the substance in which 

 they are sunk. The form of the holes may be seen by reference to the illustration, and 

 the reader will doubtlessly notice that their direction is by no means uniform ; the 

 generality being at right angles with the surface, but many arranged in an oblique di- 

 rection, and some driven almost horizontically. 



The method by which this little mollusc contrives to excavate its chamber is a com- 

 plete mystery. It is known that in its earlier stages it spins a byssus, and attaches it- 

 self to 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 cham- 

 ber 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 

 dissolving the rock ; 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 wonderful rapidity, and that, in all cases, the chamber 

 corresponds with the shape and size of the shell. It is evident, also, that the shell it- 

 self 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 

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



