38 



LYELL'S ELEMENTS OF GEOLOGY. 



Gradual Deposition indicated by Fossils. 



in which single fossils may sometimes throw light on a former 

 state of things, both in the bed of the ocean and on some ad- 

 joining 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 dug out, drilled all over by the holes of these 

 borers, the tubes and shells of the moUusk still remaining in the 

 cylindrical hollows. In Fig. 13., e, a representation is given of 

 a piece of recent wood pierced by the Teredo navalis, or com- 

 mon ship-worm, which destroys wooden piles and ships. When 



Fig. 12. 



Fossil and recent wood drilled by perforating mollusca. 



Fig. 12. a. Fossil wood from London clay, bored by Teredina. 



b. Shell and tube of Teredina personata, the right hand figure the vert- 

 tral, the left the dorsal view. 

 Fig. 13. e. Recent wood bored by Teredo. 



d. Shell and tube of Teredo navalis, from the same, 

 f ^ c. Anterior and posterior view of the valves of the same detached from 



the tube. 



the cylindrical tube d has been extracted from the wood, a shell 

 is seen at the larger extremity, composed of two pieces, as shown 

 at c. In like manner, a piece of fossil wood (a, Fig. 12.) has 

 been perforated by an animal of a kindred but extinct genus 

 called Teredina by Lamarck. The calcareous tube of this mol- 

 lusk was united, and, as it were, soldered on to the valves of the 

 shell, (6) which therefore cannot be detached from the tube, like 

 the valves of the recent Teredo. The wood in this fossil speci- 



