410 THE MEDALS OF CREATION. Cuap. XI. 
nearly at right angles, to the mammillated projections. 
These blocks are, in truth, the stony casts of cavities formed 
by Pholades, in masses of wood, both the vegetable structure, « 
and the shells, having perished. > 
In the White Chalk specimens of this kind are occasion- 
ally found. | 
A remarkable fact, relating to some of the specimens 
from the Iguanodon quarry, remains to be mentioned. Upon 
breaking off the projections, to ascertain if any traces of 
the shells of the Pholades remained, we discovered in 
several, near the apex, a univalve shell, a species of Wertta. 
Lign. 166, fig. 6, represents a fragment of stone with two of 
the casts, which have been broken, and in each, at a, a 
univalve is imbedded. At }, the ligneous structure of the 
original wood is visible. The only hypothesis that will 
account for the appearance of these univalves in their 
present position, is that of supposing that the Nerites 
crawled into the cavities made in the mass of timber, after 
the shells of the Pholades had been removed ; and that the 
wood became imbedded in a sand-bank, and the univalves 
enclosed in the cavities; the ligneous structure in a great 
measure perished, and the stony casts of the perforations of 
the borers, with the imprisoned univalves, remained. The 
Nerites, as shown in the example figured, do not occupy 
any particular position in the tubes; one has the apex 
towards the end of the cavity, and the other lies in a trans- 
verse direction.* 
TrrEepo. Ly. p. 24.—It will be convenient to notice 
in this place another genus of boring shells, whose fossil — 
remains are far more abundant than those of the Pholas. 
The Yeredo navalis, or Ship-worm, which is the most ver- 
* In a fragment of a perforated column, from Puzzuoli, in my pos- 
session, by favour of Sir Woodbine Parish, there were numerous 
living univalves in the cavities made and previously inhabited by the 
lithodomi. 
