Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/28

18 means uniform, varying in almost every individual; and even the two valves are dissimilar to each other, as is frequently the case with boring-shells.

We will now suppose that the reader has dredged up a quantity of the bed of the sea, in which have burrowed a number of the Gastrochæna. On examination of the mass, it will be found that the molluscs have not merely bored a hole, but that they have cemented together the sand, bits of shell, stone, and other materials, and have formed from them a curiously shaped tube. A very perfect specimen of one of these tubes is shown at fig. 2. It is flask-shaped, and has a very long neck, which is curved and divided into two portions, something like the proboscis of an elephant. The double tubes, however, are not quite distinct, but communicate with each other at their junction.

It is on account of the flask-like shape of the tube that the shell has derived its specific name of modiolina, the Latin word modiolus signifying a little flask. So perfect a specimen is, however, seldom found; and although the interior of the tube is always flask-shaped, its exterior is mostly irregular, so that its real character would not be recognised except by a practised eye. In a specimen now before me, for example, the tube is of nearly the same thickness throughout its entire length, and the extremity is, if anything, larger than the base, on account of a serpula tube which is attached to it.