74 REPORT—1849. 
Similar evidence may repeatedly be seen where Pholas parva bores into chalk: the 
depression between the posterior margin of the valves occupied by the hinge cor- 
responds with a ridge in the matrix, a circumstance which it was impossible could 
have occurred if the animal had rotated. 
Opposing thus all mechanical action, the author resumed the idea of a solvent, but 
was of opinion that the solvent should be looked for in the element in which the ani- 
mals exist, and not in the resources of the animal itself. He presumed that it would be 
found in the presence of the free carbonic acid held in solution by sea-water; the ceco- 
nomy of the boring animals being simple and uniform throughout creation, the solvent 
only being directed by them according to their habits, through the process of re- 
spiration and ciliary currents. 
He next proceeded to show the action of sea-water upon limestone coasts, attributing 
their peculiar appearance to the presence of carbonic acid in the sea-water ; and upon 
rolled limestone pebbles, particularly those which had been previously bored by small 
annelides, which he presumed gave a passage for sea-water to the centre of the stone, 
and tended to wear them rapidly and irregulariy, evidence of which may be frequently 
seen in those which have been perforated by sponges, becoming half-buried in sand, 
where the exposed side is corroded, whilst the protected part remains uninjured. 
The author’s opinion was, that the boring of Saxicava was to be attributed to the 
same means, the animal only causing the solvent to act more uniformly. He 
believed that the perforations of Saxicava were the work of time, and his experience 
went to show that the animal could not bore deeper into the rock after it had lost the 
power of locomotion, which was very early in its existence; after which, excavation 
only continued to enlarge the cavity so as to adapt it to the increasing buik of the 
animal, as well as the entrance to the excavation, which in the young barely exceeds 
the sixteenth of an inch, while in that of the adult it often equals the diameter of the 
animal, and always bore a corresponding ratio to the thickness of the shell; thick- 
ness of shell denoting age, and with years the diameter of the bore inereases, 
With regard to the Pholas tribe, he presumed that they penetrate soft clays through 
a modification of the same power; that is, in all the Lithophagi the caleareous rock 
is dissolved by the carbonic acid in the currents induced by the animal, but in that 
of the borers into clay, the wearing power is the mechanical action of those currents, 
which ave greatly increased by the muscular power of the animal; the carbonic acid 
still taking up any caleareous matter which may be present, as in the case where they 
are found to bore in soft triassic sandstone. In this opinion he considered that he was 
supported by the observations of Mr, Osler, who in explaining how (what he presumes 
to be) the rasped waste is expelled during the process of excavation, says, ‘‘ When the 
projected syphon is distended with water, the Pholas closes the orifices of the tubes 
and retracts them suddenly ; the water which they contain is thus ejected forcibly from 
the opening of the mantle, and the jet is prolonged by the gradual closing of the 
valves to expel the water contained within the shell; the chamber occupied by the 
animal is thus completely cleansed.” ‘That the Pholas expels material by the force of 
currents, is shown in this passage, but it is only hypothetical that it had been pre- 
viously rasped off by the shell. The foregoing evidence shows that Pholas cannot 
rotate within its cavity ; consequently the waste, seen to be expelled by Mr. Osler, 
could not have been first rasped off; therefore it is not unfair to presume that it was 
worn off by the mechanical force of the current excited by the animal. It is only by 
presuming such to be the case that we can account how Pholas candida ean be found 
to bore into clay and peat on the coast of Wales, pure sand at Exmouth, and lias at 
Lyme Regis. 
The remarkable manner in which shells and rolled pebbles are perforated by Cliona, 
will easily, the author presumed, be explained by the same theory. He argued that 
a sporule of Cliona (which is a true sponge in all its conditions) first obtains a footing 
in some crevice, where it developes itself so as to penetrate the whole fabric, destroying 
the shell or pebble by simply fulfillmg the condition of its existence, which is by 
pouring its currents ina given direction, until a passage be broken through by the 
corroding power of the carbonic acid in those currents. He mentioned a case which fell 
under his observation, in which a Saxicava was not only checked but turned aside and 
deformed by coming into contact with Cliona. Neither, from their own power, were 
eapable of effecting a passage through the other. The sponge from its nature could not 
be acted upon by the solvent, which it is presumed the mollusk uses; neither could 
— 
Cie 
