Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin in the chalk (see fig. 13.), which has fixed to it the lower valve of a _Crania_, a genus of bivalve mollusca. The upper valve (_b_, fig. 13.) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. Then the young _Crania_ adhered to the bared shell, grew and perished in its turn; after which the upper valve was separated from the lower before the _Echinus_ 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 dug out, drilled all over by the holes of these borers, the tubes and shells of the mollusk still remaining in the cylindrical hollows. In fig. 15. _e_, 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 _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. 14.) has been perforated by an animal of a kindred but extinct genus, called _Teredina_ by Lamarck. The calcareous tube of this mollusk was united and as it were soldered on to the valves of the shell (_b_), which therefore cannot be detached from the tube, like the valves of the recent _Teredo_. The wood in this fossil specimen is now converted into a stony mass, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the _Teredinae_ lived upon it, perforating it in all directions. Again, before the infant colony settled upon the drift wood, the branch of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate. [2 Illustrations: Fossil and recent wood drilled by perforating Mollusca. Fig. 14. _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. 15. _e_. Recent wood bored by _Teredo_. _d_. Shell and tube of _Teredo navalis_, from the same. _c_. Anterior and posterior view of the valves of same detached from the tube.]