STONE BORERS. 297 
animals, and soon commence to perforate. Both 
that gentleman and Mr. Garner have noticed that 
their excavations are not round, nor the sides 
smoothed off, like those of the holes made by 
Pholas. As for us, we only know of their boring 
into caleareous rocks, but Mr. Clarke has noticed 
an instance of their perforating triassic sandstone 
at Exmouth. Wherever we have a sea-coast of 
mountain limestone, the surface of the rocks is 
almost invariably found riddled by Saxicava. The 
whole front of the Plymouth breakwater has been 
attacked by it, and much alarm for its safety ex- 
cited. Mr. Couch observes that the Sazxicava 
never bores deeper than six inclies, and that, con- 
sequently, unless a new surface be exposed by 
the destruction of the perforated part, there 1s not 
much danger. Owing, however, to the thinness 
of the partitions, which often are the only separa- 
tion between the crypts of these mollusks, there 
is a great probability of the action of the sea 
rapidly forming new surfaces in such cases. How 
they bore has been as much discussed as the 
question how Pholas bores. The general opinion 
has been, that Sazicava bores by means of an 
acid secretion ; an opinion held by many who will 
not admit the probability of such an agent being 
used by the Pholadide. My. Osler, though inclined 
to such a view, could detect no acid, nor, for 
reasons previously stated, is it likely. Mr. Han- 
cock, as we have seen when treating of Pholas, 
expressly asserts that the Saxicave bore by rasp- 
ing, effected by means of siliceous particles con- 
tained in the anterior part of the mantle. Mr. 
Couch entertains a similar view. We have not 
been able to satisfy ourselves of the presence of 
