Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/33

Rh discover some spot on our coast which was rich in shells hitherto excluded from the British list, and yet be entirely mistaken if he were to consider them as true inhabitants of our shores. The fact is, that great quantities of shells are often conveyed from one country to another among the ballast, and when the sailors throw away the ballast overboard, they also fling into the sea various shells among the stones and sand. These shells are subsequently washed up by the tides, or dashed on the shore in a storm, so that they are picked up by hand, or inclosed in the multifarious contents of a dredge. Sometimes, too, a ship in ballast is cast upon the shore and beaten to pieces by the waves, when the ballast is necessarily thrown out, and in a year or two becomes a part of the shore. In this way many enterprising collectors have been deceived, and their mistake has not been discovered until many years afterwards.

We are now come to some shells that are popularly and appropriately called Gapers, because their valves are not wholly closed, but are permanently open at each end. The reader will remember that a similar structure is seen in the Flask-shells, as well as in others which have already been described, so that they all have a partial right to the name.

The species which is most generally known is the, or (Mya arenaria); it is shown at Plate II., fig. 6. As it is rather a large species, the figure is necessarily reduced in size. The chief peculiarity of the Gaper-shells is seen at a glance, namely, a membranous tube which projects from the upper opening of the shells, and which contains the siphons. This tube is very thin, and is, in fact, the prolongation of a membrane which covers the entire body and the greater part of the shell, and which is technically named the "epidermis." Towards the hinder part of the shell the membrane is very thin, and at last is almost imperceptible; but in front it is