where the two substances are mixed, and its empty valves are often found upon the shore.
The animal is remarkable for the length and mobility of its siphon tubes. These organs are slender, unconnected except at their bases, elastic, and are capable of being extended until they equal the length of the shell. They can also be expanded at will, until they are at least three times their usual size. Like the corresponding organs in several other shells, they are covered with a thin membranous tube. About four species of Abra-shells are known to be British.
There is another curious shell which is clearly related to the Abra-shells, and which is generally found in the mud at the mouths of tidal rivers. This is the species called scientifically Scrobicularia piperita, a name which we may translate almost literally by Furrow-shell. It derives its name from the numerous deep but narrow furrows which run round the shell, and which prevent it from assuming any gloss.
Many of the previously-mentioned shells are very variable in form and colour, but this species is of so uncertain a form, that it has been separated into various species by different conchologists. When unstained by the mud in which they have rested, the valves are yellowish white on the outside, sometimes having a slight tinge of orange. The interior is white and glossy, sometimes tinged with yellow, and the hinge is formed on a similar plan to that which has been so often mentioned. A drawing of the hinge may be seen at page 25, fig. 14.
This is a very plentiful species, and as it has a love for brackish water, it may be found at some distance from the mouth of the river, so that it is exposed to the alternation of salt, brackish, and nearly fresh water. As it lives at a considerable depth in the mud, it is seldom taken in a living state except by those who search for such objects; and, as a general rule, the specimens which are seen in ordinary cabi-
