ON A PIECE OF CHALK 65 



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 embedded a little higher up have, 

 in like manner, lived and died. But some of these remains 5 

 prove the existence of reptiles of vast size in the chalk sea. 

 These lived their time, and had their ancestors and descend- 

 ants, which assuredly implies time, reptiles being of slow 

 growth. 



There is more curious evidence, again, that the process 10 

 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; 15 

 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 20 

 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. 25 



"The upper valve is almost invariably wanting, though occasionally 

 found in a perfect state of preservation in the white chalk at some dis- 

 tance. 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 Crania adhered to the bared shell, grew and perished 30 

 in its turn ; after which, the upper valve was separated from the lower, 

 before the Echinus became enveloped in chalky mud." 1 



A specimen in the Museum of Practical Geology, in London, 

 still further prolongs the period which must have elapsed 



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



