50 
ELEMENTS OF GEOLOGY. 
lived from youth to age, then died and lost its spines, which 
were carried away. Then the young Crafiia adhered to the 
bared shell, grew and perished in its turn; after which the 
upper valve was separated from the lower before the Anan- 
chytes became enveloped in chalky mud. 
It may be well to mention one more illustration of the 
manner in which single fossils may sometimes throw light 
on a former state of things, both in the bed of the ocean and 
on some adjoining land. We meet with many fragments of 
wood bored by ship-worms at various depths in the clay on 
which London is built. Entire branches and stems of trees, 
several feet in length, are sometimes found drilled all over 
by the holes of these borers, the tubes and shells of the mol- 
lusk still remaining in the cylindrical hollows. In Fig. 14, 6, 
Fig. 13. 
Fossil and recent wood drilled by perforating Mollusca. 
Fig. 13. a. Fossil wood from London clay, bored by Teredina. 
b. Shell and tube of Teredina personata, the right-hand figure the ventral, 
the left the dorsal view. 
Fig. 14. e. "Recent wood bored by Toredo. 
d. Shell and tube of Teredo navalis, from the same. 
c. Anterior and posterior view of the valves of same detached from the tube. 
a representation is given, of a piece of recent wood pierced 
by the Teredo navalis^ or common ship-worm, which destroys 
wooden piles and ships. When the cylindrical tube c? has 
been extracted from the wood, the valves are seen at the 
larger or anterior extremity, as shown at c. In like man¬ 
ner, a piece of fossil wood (a. Fig. 13) has been perforated 
by a kindred but extinct genus, the Teredina of Lamarck. 
The calcareous tube of this mollusk was united and, as it 
were, soldered on to the valves of the shell (5), which there¬ 
fore can not be detached from the tube, like the valves of 
