Serpula attached to a fossil Spatangus from the chalk.
Fig. 12.
Recent Spatangus with the spines removed from one side.
- b. Spine and tubercles, nat. size.
- a. The same magnified.
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.
Fig. 13.
