60 ON A PIECE OF CHALK 



cess 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 Mncovered upon 

 the sea-bottom long enough to lose all its outward cover- 

 ings and appendages by putrefaction; and that, after 

 this had happened, another animal might attach itself 

 to the dead and naked skeleton, might grow to matu- 

 rity, 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 pre- 

 servation 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, which 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." 



A specimen in the Museum of Practical Geology, in 

 London, still further prolongs the period which must 

 have elapsed between the death of the sea-urchin, and 

 its burial by the Globigerince. For the outward face of 

 the valve of a Crania, which is attached to a sea-urchin 

 (Micraster), is itself overrun by an incrusting coralline, 

 which spreads thence over more or less of the surface 

 of the sea-urchin. It follows that, after the upper 



