ON A PIECE OF CHALK 77 



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 thdir 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 putre- 

 faction; 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 described 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. 



"The upper valve is almost invariably wanting, though 

 occasionally found in a perfect state of preservation in the 

 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, w^hich 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." ^ 



1 Elements of Geology, by Sir Charles Lyell, Bart., F.R.S., p. 23. 



