Page:The common shells of the sea-shore (IA commonshellsofse00wood 0).pdf/36

26 valve of a Gaper-shell, and just below the "umbo," or boss of the hinge, is placed the peculiar structure which has been mentioned. At fig. 2 is drawn the hinge of the left valve, on which is a strong piece of horny and very elastic substance, shaped something like one of the fans of a screw propeller. The hinge of the right valve is seen at fig. 1, where is shown the projecting portion which corresponds to the horny fan. This projection is of considerable length, is very strong,, and highly polished and smooth on the under side; the upper side being comparatively rough and slightly wrinkled. When the two shells or valves are inhabited by the animal, the shelly projection presses against the horny fan, which acts as a spring, and gives the shells a constant tendency to fly open, which is only counteracted by the force of the powerful muscles which close the valves, and which leave a deep impression of their attachment upon the interior of each valve. This curious apparatus holds the valves apart, much as the spring of a pair of shears keeps their blades open.

Just below the Gaper-shell is drawn another species, which we may term the (Mya truncata). This species derives its name from the shape of the shell, which looks as if it had been abruptly cut off or squared at the end from which the siphons protrude. In this figure the animal is shown as it appears when in health and unalarmed, the siphons with their fringed ends projecting from the end of the tube.

Those who really desire to study the marine shells of England, ought to procure some living specimens of the Gaper-shells. Both species can often be found in the same locality, and dug out of their muddy homes without much labour. The structure of the animal is well worthy of examination, as the creature forms one of the simplest examples of the molluscs, and becomes a key to the more complicated anatomy of other species. When the valves are opened, the reseru-