PART I. CHAPTER III. 



37 



Gradual Deposition indicated by Fossils. 



Fig. 9. 



. 10. 



that two serpulse have grown on the interior, one of them ex- 

 actly on the place where the adductor muscle of the Gryphtra 

 (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 crea- 

 ture 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. 10. a 



living species of Spa- 

 tangus, common on 

 our coast, is repre- 

 sented with one-half 

 of its shell stripped 

 of the spines. In Fig. 

 9, a fossil of the same 

 genus from the white 



Recent Spatangus7witk the chalk of England 

 spines removed from one shows the naked Slir- 



&.tp!ne and tubercles, f *<* which the > indi- 

 natural size. Vlduals of this family, 



a. The same magnified, exhibit when denuded 

 of their bristles. The full-grown Serpula, therefore, which now 

 adheres externally, could not have begun to grow till the Spa- 

 tangus had died, and the spines were detached. 



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. 11.), which has fixed to 

 it the lower valve of a crania, an extinct genus 

 of bivalve mollusca. The upper valve (6 Fig. 

 11.) is almost invariably wanting, though occa- 

 sionally 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 

 a. Echinus from the Crania adhered to the bared shell, and perish- 



va?ie' onSJ ^ranTa ed in its turn ? after which the U PP er Valve Was 



attached. separated from the lower before the Echinus 



' cSia detached. the became enveloped in chalky mud. 

 It may be well to mention one more illustration of the manner 

 D 



Serpula attached to 

 fossil Spalangus 

 from the chalk. 



