112 



in consequence of the fragility of the inclosed body being such, that 

 it is almost always very much shattered by the blow, which is neces- 

 sary to break the pebble which incloses it. In the mean time, the re- 

 gularity and constancy "of the form which it bears in the multitude of 

 its specimens which may be met with, is no small proof of its having 

 derived its form from some organized being. 



The constant uniformity of the figure Avhich this substance bears, 

 is by no means offered as a proof alone of its animal origin ; since, as 

 in the instance of the Ketton and other stones, a constant uniformity 

 of figure is observable, without any reason appearing for supposing 

 them ever to have existed but as minerals. A circumstance, however, 

 is observable in these pebbles, as has been already slightly noticed, 

 which at least proves three distinct stages in their formation ; and 

 also tends to confirm the suspicion of the inclosed body having existed 

 in a distinct state. The openings in the pebble, through which 

 the processes have passed, plainly shew, that this,, body must have 

 previously existed, and that it has been a nucleus, in which the sur- 

 rounding silicious matter has been deposited. Those marks in the 

 pebble, too, where traces only of these openings can be seen, the open- 

 ings themselves being filled up by silicons matter, shew that an ad- 

 dition of this silicons matter, in a fluid state, has been made, after 

 the pebble had been formed round this nucleus, and after the project- 

 ing bodies, which had filled those openings, had been removed by 

 some force. 



At Fig. 14, a representation is given of the appearance which some 

 pebbles derive from this body. It is at the wide superior extremity, 

 where the processes have passed, that meandering lines are disco- 

 vered. These seem to have been formed by the concurrence of the 

 edges of two or three openings, which have been filled up by silicious 

 matter. 



I am fully aware of the scepticism with which my conjectures on the 

 nature of the preceding fossil will be received : that a substance so lit- 



