I ON A PIECE OF CHALK 21 



when the place which they now occupy was the 

 surface of as much of the chalk as had then been 

 deposited ; and that each has been covered up by 

 the layer of Globigerina mud, upon which the 

 creatures imbedded a little higher up have, in 

 like manner, lived and died. But some of these 

 remains prove the existence of reptiles of vast 

 size in the chalk sea. These lived their time, 

 and had their ancestors and descendants, which 

 assuredly implies time, reptiles being of slow 

 growth. 



There is more curious evidence, again, that the 

 process of covering up, or, in other words, the 

 deposit of Globigerina, skeletons, did not go on 

 very fast. It is demonstrable that an animal of 

 the cretaceous sea might die, that its skeleton 

 might lie uncovered upon the sea-bottom long 

 enough to lose all its outward coverings and 

 appendages by putrefaction ; and that, after this 

 had happened, another animal might attach itself 

 to the dead and naked skeleton, might grow to 

 maturity, and might itself die before the calcareous 

 mud had buried the whole. 



Cases of this kind are admirably desciibed by 

 Sir Charles Lyell. He speaks of the frequency 

 with which geologists find in the chalk a fossilized 

 sea-urchin, to which is attached the lower valve of 

 a Crania. This is a kind of shell-fish, with a shell 

 composed of two pieces, of which, as in the oyster, 

 one is fixed and the other free. 



