FROM THE CHALK. 41 



exact mould of the cavity of the shell. The Echlnites sim- 

 ply filled with flint, after being torn from their bed of chalk 

 by the operation of diluvial currents, are worn away by boul- 

 dering, and the internal cast becomes exposed ; — the disin- 

 tegrated silex, (no longer protected by the shelly covering), 

 then separates from the rest of the mass, leaving the hollows 

 or deficiencies that I have described. I do not pretend to 

 explain how the disintegration of the flint originates, or to 

 define the change in its mineral condition, as that enquiry 

 forms a distinct subject of investigation. 



Partial silicljlcation of the Shell Itself. — The above re- 

 marks have reference solely to the flint which fills the cavity 

 of the shell ; but the shell itself is frequently more or less 

 silicified, in this respect following a general law which ap- 

 pears to me altogether inexplicable, namely, the shells which 

 have undergone the greatest amount of silicification, are those 

 which contain false casts ; it is comparatively rare for the 

 shells containing true casts, to exhibit any traces of this 

 process. 



The silicifying process appears to commence at a short 

 distance from the natural openings of the shell, and a beau- 

 tifully defined siliceous ring having been deposited around 

 each of the ambulacral perforations, and also around the 

 mouth and vent, the silicification frequently does not extend 

 farther. At other times the silex proceeds to invest the se- 

 creting membrane of the shell, where this membrane dips 

 between the sutures of the separate plates; and instances 

 sometimes occur where one third or more of the calcareous 

 matter is replaced by silex. 



Cavity of the Shell not always perfectly filled, — It some- 

 times happens that the silex does not fill the entire cavity of 

 the Echinus ; but I believe that when this is the case, it in- 

 variably lines the whole internal surface of the shell, leaving 

 a central hollow, the interior of which presents a chalcedonic 

 or crystallized surface. The nature of this surface therefore 

 readily shows, whether the deficient part of the mould arises 

 from the cavity never having been filled, or from the subse- 

 quent process of disintegration. The occasional imperfect fill- 

 ing of the cavity of the Echini, is a condition common to 

 both the true and the false casts. 



I have never yet met with an Echinus enveloped exteraally 

 with flint, having chalk in its interior, a circumstance suffi- 

 ciently curious to deserve notice. 



Distinct Crystals of Calcareous Spar on the internal sur- 

 face of the Shell. — It is by no means a circumstance of rare 

 occurrence to find crystals of calcareous spar lining a portion. 



Vol. IV.— No. 37, n. s. g 



