' 306 DIMYARIA.—PHOLADID. 
gratuitously so; for we can hardly suppose that 
any of those who have taken this view of the 
cause would maintain, that the animals have the 
power of secreting different acids at will, according 
to the substance they have ‘to attack. Yet this 
notion has been most favoured by naturalists, who, 
sceptical as to the perforating power of such fragile 
instruments as are the shells of many of these crea- 
tures, endowed the animals with supernatural che- 
mical qualifications. Even good experimental 
observers—Mr. Osler for one—whilst they proved. 
that the Pholas could bore mechanically by the 
rotation of its valves, could not free their minds 
from the prejudice in favour of a solvent. The 
important statement put forward by Mr. Albany 
Hancock, respecting the instruments by which 
Mollusca bore, and which, so far as Gasteropoda 
are concerned, appear to furnish us with a true 
explanation,—namely, that it was effected by 
means of silicious particles, variously arranged in 
certain portions of the animal’s body,—led us to 
hope that a better cause than any yet alleged had 
been discovered. But we cannot bear it out with 
respect to the Pholadide. We can find no such 
particles in the mantle of the Teredo, nor have any 
been noticed by Home or Deshayes, or by the most 
recent observers, Fray and Leuckart, who paid 
especial attention to the structure of the tissues in 
this genus. Nor could we, although aided by the 
anatomical and microscopical skill of Mr. Busk, 
detect any silicious particles in either the mantle, 
foot, or siphon tube of Pholas candida. If present 
in any species, therefore, they are exceptional, so 
far as the genus Pholas and its allies are concerned. 
The shells of several British species of Pholas, and 
