and boring marine animals . 367 
occasionally Mya truncata, in similar situations. These are 
the only circumstances under which I have found the Litho- 
phagi inhabiting insoluble stones, and such an exception con- 
firms the general rule. 
I have stated, that where the Saxicavae are abundant, their 
holes very often communicate ; and that their shells are 
acted on when exposed to the foot of another. On exa- 
mining a considerable number, taken indiscriminately from 
the same rock, I have found that the shells of more than 
half had been thus injured. As long as the injury is super- 
ficial, no attempt is made to repair it ; but when the shell is 
nearly or quite penetrated, the breach becomes filled, not 
with new shell, but with a firm yellow substance, which is 
insoluble even in a strong mineral acid. It would be difficult 
to conceive a fact short of absolute demonstration, which 
could give a more decisive support to the theory of a solvent. 
A peculiar provision is given to the animal to preserve it 
from destruction by an injury, to which it is particularly 
exposed. The supposition of mechanical penetration would 
require us to believe that a newly formed substance, much 
softer than that which has been destroyed, can stop the pro- 
gress of the mischief, and even repair it, under the continued 
application of the original destructive force. 
I have delayed the communication of this paper for many 
months, from a wish to complete the subject by detecting 
the solvent ; but every experiment I have made for this 
purpose has been quite unsuccessful. Litmus paper applied 
to every part of Saxicavae just taken from their holes, shells 
stained, and sea- water tinged with the same test, in which 
animals of every size were kept till they died, gave no 
