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Floating wood, containing perforating shells, such as Pholas and Gastrochoena, occur. There are likewise a few beds of a yellowish-brown limestone, with marine shells, which enable us to prove that the lowest and highest plant-beds belong to one group.
"The `pholas dactylus, as scientific people call it, which, until Gosse, as I said, discovered its mode of action, was quite a puzzle to every one; although, now that the mystery is out, all wonder it was not cleared up before! If you look at the head of the shell, you'll see it is provided with a regular series of little pointed spines at the end of the upper portion.
There are certain rock-boring animals, such as the Pholas, which help to decay the rocks. Each mollusc cuts a series of augur-holes from two to four inches deep, and so assists in destroying the bulwarks of England. Atmospheric action, the disintegration of soft rocks by frost and by the attack of the sea below, all tend in the same direction.
But it is not merely after death that Pholas becomes luminous a phenomenon perfectly familiar even in the case of many fish, especially the herring and mackerel.
'The Pholas dactylus' he says, 'makes its hole by grating the chalk with its rasp-like valves, licking it up, when pulverised, with its foot, forcing it up through its principal or bronchial syphon, and squirting it out in oblong nodules.
It is large and pointed, and bent: if the Cockle wishes to move quickly, it stretches out its foot from between the shells, as far as it will go. Then, by using all its power, it leaps backwards or forwards in a surprising manner. There are many other interesting molluscs, besides those we have looked at. The Piddock, or Pholas, is a smallish, rather delicate one, with a soft foot.
In Pholas the light is still more persistent, and it is found that when the dead body of this mollusk is placed in honey, it will retain for more than a year the power of emitting light when plunged in warm water.
He adds that on the giant specimen just referred to be wrote his name with his finger as it lay on the deck in a tub at night, and in a few seconds he had the gratification of seeing his name come out in "letters of fire." Among mollusks, the best known instance of phosphorescence is in the rock-boring Pholas, the luminosity of which after death is mentioned by Pliny.
This species of Pholas still exists, and drills the rocks between high and low water on the British coast. The name of "Fluvio-marine" has often been given to this formation, as no less than twenty species of land and fresh-water shells have been found in it.
The crypt protects the Pholas from confervæ, which, when they get at it, grow not merely outside, but even within the lips of the valves, preventing the action of the syphons. In the foot there is a gelatinous spring or style, which, even when taken out, has great elasticity, and which seems the mainspring of the motions of the Pholas dactylus.
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