365 
and boring marine animals. 
the assailant. When the animals meet laterally, they become 
compressed and distorted in the course of their growth, but 
their shells are never injured. 
If the Lithophagi perforate mechanically, the shell and the 
foot are the only instruments which they can possibly em- 
ploy. It has been shown that the shell is not adapted, or 
even required, for this purpose ; and it must be quite unne- 
cessary to attempt to prove that a stone cannot be rubbed 
away by a mere vesicle, like the distended foot ; especially 
when it is observed that the syphon, which is so much thicker 
and stronger, is preserved from the effects of friction by a 
particular strong cuticle. To ascertain that the mechanical 
powers of the animal are quite unequal to make an impres- 
sion upon a stone, would of itself almost justify the presump- 
tion that the task is effected by a solvent. 
But the theory of a solvent does not rest on mere negative 
proof. It appears impossible to explain, on any other grounds, 
the animal’s exclusive choice of calcareous matter; a choice 
evidently depending on its inability to penetrate stones of a 
different nature. I have already alluded to specimens, in 
which silex projects into their holes, and in which a very small 
stratum of insoluble stone has arrested the progress of 
the Saxicava. An exception may perhaps be taken to the 
hardness of the silex, though it certainly is not harder 
than the stone in which it is imbedded ; but I am unable to 
perceive how the fact connected with the other specimen is 
to be explained away. It may be observed too, that in this 
specimen the holes, instead of being smooth, and almost po- 
lished, as usual, are uniformly rough ; presenting, when exa- 
mined under a glass, precisely the fretted appearance which 
MDCCCXXVI. 3 B 
