DEPOSITION INDICATED BY FOSSILS. 
49 
oyster or other shell while the animal within was still living; 
but if they are found on the inside, it could only happen after 
the death of the inhabitant of the shell which affords the sup¬ 
port. Thus, in Fig. 9, it will be seen that two serpulae have 
grown on the interior, one of them exactly on the place where 
the adductor muscle of the Gryphcea (a kind of oyster) was 
fixed. 
Some fossil shells, even if simply attached to the outside 
of others, bear full testimony to the conclusion above al¬ 
luded to, namely, that an interval elapsed between -the 
death of the creature to whose shell they adhere, and the 
burial of the same in mud or sand. The sea-urchins, or 
Echini^ so abundant in white chalk, afford a good illustra¬ 
tion. It is well known that these animals, when living, are 
invariably covered with spines supported by rows of tuber¬ 
cles. These last are only seen after the death of the sea- 
urchin, when the spines have dropped off. In Fig. 11a liv¬ 
ing species of Spatangzis^ common on ouu coast, is represent¬ 
ed with one-half of its shell stripped of the spines. In 
Fig. 10 a fossil of a similar and allied genus from the white 
chalk of England shows the naked surface which the indi- 
Fig. 12. 
а. Anmichytes from the 
chalk with lower 
valve of Crania at¬ 
tached. 
б. Upper valve of Cra¬ 
nia detached. 
viduals of this family exhibit when denuded of their bris¬ 
tles. The full-grown Serpula^ therefore, which now adheres 
externally, could not have begun to grow till the Micraster 
had died, and the spines became detached. 
Now the series of events here attested by a single fossil 
may be carried a step farther. Thus, for example, we oft¬ 
en meet with a sea-urchin {Ananchytes) in the chalk (see 
Fig. 12) which has fixed to it the lower valve of a Crania^ 
a genus of bivalve mollusca. The upper valve (5, Fig. 12) 
is almost invariably wanting, though occasionally found in 
a perfect state of preservation in white chalk at some dis¬ 
tance. In this case, we see clearly that the sea-urchin first 
3 
Fig. 10. 
Fig. 11. 
Serpula attached to 
a fossil Micraster 
from the chalk. 
Recent Spatangus with the spir 
removed from one side. 
b. Spine and tubercles, natui 
size. 
a. The same magnified. 
