FOSSIL PHOLADES, 409 
by its operations. Some species burrow in wood, and often 
commit serious ravages in piles and other submarine works 
constructed of timber. In the earlier ages of our planet 
"we find evidence of the existence of the same kind of living 
instruments for the disintegration of floating wood, and the 
reduction of masses of rock into detritus. But no traces of 
these shells have been found in strata below the Oolite. 
One species occurs in the Coral Rag, another in the Kim- 
meridge Clay ; two in the Galt and Green Sand; and three 
or four in the tertiary deposits. In the Crag, blocks of 
stone are occasionally found with the shells of Pholades 
occupying the perforations they originally formed and 
inhabited. But all the specimens I have observed in the 
Galt, Green Sand, and Oolite were xylophagous (wood- 
eating) species. In the Shanklin Sand, masses of fossil 
wood, literally honey-combed by the perforations of Pho- 
lades, are frequent ; but the shells themselves are rare. Mr. 
Sowerby has figured a beautiful specimen of silicified wood, 
from Sandgate, with numerous shells of this genus (Pholas 
priscus. Min. Conch. tab. 581). Lign. 166, fig. 5, repre- 
sents a fragment of fossil wood, with three shells an sitw; a, 
a shell seen longitudinally ; and below, the rounded anterior 
extremities of two other shells are exposed. 
Masses of wood perforated by Pholades, from which all 
traces of the shells have disappeared, have given rise to 
some curious fossil remains, which are often very enigma- 
tical to the young collector. In the Kentish Rag, as for 
example, in Mr. Bensted’s quarry, near Maidstone, large 
blocks of stone are found, covered with groups of subcylin- 
drical mammillary projections, which are obtuse or rounded 
at the apex. In some examples the interstices between 
these bodies are free; in others they are occupied by a red- 
dish brown, friable substance, presenting obscure indications 
of ligneous structure; and rarely, distinct woody fibres 
may be observed, the direction of which is transverse, or 
