40 SILICEOUS CASTS OF ECHINITES 



in its natural position,) without rising high enough to fill the 

 entire cavity. 



The true casts principally differ from XhQ false, in present- 

 ing us with a faithful fac-simile of the internal surface of the 

 ambulacral plates ; and unless worn smooth by bouldering, 

 or other causes, the course of the ambulacra is indicated by 

 rows of short, cylindrical, siliceous processes, which are, in 

 fact, nothing but the casts of the ambulacral pores ; and the 

 length of these processes is consequently just that of the 

 thickness of the original shell. These processes upon the 

 true casts, conespond to the pits upon the surface of the false 

 casts. 



Now, the presence of the siliceous processes, marking the 

 course of the ambulacra, is a circumstance that we should 

 naturally expect, the anomaly consists in this condition being 

 so generally reversed, — a hollow taking the place of d^ projec- 

 tion. Every collector of fossils has probably noticed the dif- 

 ference in the two conditions, but I am not aware that any 

 attempt has yet been made to explain how the difference 

 originates. The secret of the matter is simply this : — In the 

 case of the true casts, the silex has not only filled the 

 cavity of the shell, but it has also completely enveloped the 

 shell externally, the Echinus forming as it were, the nucleus 

 of the flint nodule. In the false casts the siliceous matter 

 has filled the cavity of the shell, but not surrounded it with 

 a mass of the same substance. Now in both these instances, 

 the cavity of the Echinus is entirely filled with silex, and 

 the difficulty therefore is still unaccounted for ; but if we 

 take an Echinus filled with, but not surrounded by, flinty 

 matter, and by an artificial process carefully remove the 

 shell, it will be found, that a portion of the contained silex 

 (forming the cast) is in a state of disintegration. Now it 

 seems, that this process of disintegration always commences 

 on those portions of the casts which are in the immediate 

 neighbourhood of the natural openings of the shell, and that 

 it goes forward to a greater or less extent, without the degree 

 being regulated by any apparent law * ; but where the Echi- 

 nus forms the nucleus of a mass of flint, the disintegrating 

 process takes place ( if at all ) on the surface of the entire 

 stone, and does not reach the flint within the Echinus ; con- 

 sequently, the casts formed in this way, present us with an 



' I have used the term " disintegrated" silex, in the absence of a more 

 appropriate designation. That this condition of the silex is an altered one 

 and not the state in which it was originally deposited, I have in my pos- 

 session tolerably conclusive evidence. 



