In like manner, when we see thousands of full-grown shells dispersed every where throughout a long series of strata, we cannot doubt that time was required for the multiplication of successive generations; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with serpulae, or barnacles (acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the testacea, now adhering to it, grew from an embryo state to full maturity. Attached shells which are merely external, like some of the serpulae (_a_) in the annexed figure (fig. 10.), may often have grown upon an 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 support. Thus, in fig. 10., 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 _Gryphaea_ (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 alluded 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 illustration. It is well known that these animals, when living, are invariably covered with numerous spines, which serve as organs of motion, and are supported by rows of tubercles, which last are only seen after the death of the sea-urchin, when the spines have dropped off. In fig. 12. a living species of _Spatangus_, common on our coast, is represented with one half of its shell stripped of the spines. In fig. 11. a fossil of the same genus from the white chalk of England shows the naked surface which the individuals of this family exhibit when denuded of their bristles. The full-grown _Serpula_, therefore, which now adheres externally, could not have begun to grow till the _Spatangus_ had died, and the spines were detached. [Illustration: Fig. 11. _Serpula_ attached to a fossil _Spatangus_ from the chalk.] [Illustration: Fig. 12. Recent _Spatangus_ with the spines removed from one side. _b._ Spine and tubercles, nat. size. _a._ The same magnified.] [Illustration: Fig. 13. _a._ _Echinus_ from the chalk, with lower valve of the _Crania_ attached. _b._ Upper valve of the _Crania_ detached.]