190 UTag Sjcrntmts, (Essags, antr Itattefas. [ix. 



strable 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 appen- 

 dages 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 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." 1 



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 valve of the Crania fell 

 off, the surface of the attached valve must have remained 

 exposed long enough to allow of the growth of the whole 

 coralline, since corallines do not live imbedded in mud. 



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



