FROM THE CHALK. 39 



connected with the results now indicated. On the contrary, 

 I should be much gratified if the hints contained in this short 

 notice, were to form the basis of a thorough examination of 

 the whole matter, by any one who may have the time at com- 

 mand, and the necessary materials within reach, for following 

 up the enquiry. 



Circumstances attending the disappearance of the Shell 

 from the investing siliceous Matrix. — Upon breaking up the 

 masses of common flint which have been taken from the chalk, 

 where that substance is quarried for economic purposes, the 

 contained shells of the Echinites, or the calcareous spar re- 

 presenting the shell, will be found entire, and agreeing with 

 the shells which occur in the chalk itself. This, however, 

 is not the case with the chalk-flints that, at some remote pe- 

 riod, have been removed from their original site, and sub- 

 jected to diluvial action. In these latter, the shells of the 

 Echini have disappeared, the removal being either total or 

 partial, according to the alteration in character which the in- 

 terior of the flint exhibits. In the ordinary flint-gravel, as 

 for example that at Household Heath, near Norwich, the ori- 

 ginal aspect of the flint is exchanged for a grey or a brownish- 

 yellow colour, and then the calcareous matter of the included 

 fossils is entirely gone, and the space which it occupied left 

 quite free. But in other places, beds of flint may be found 

 overlying the chalk, in which the change in the original con- 

 dition of the silex is but slight, and the shells of the Echini 

 and other fossils are then only partially removed. 



Proposed separation of the flint Casts into true and false. 

 — The internal siliceous moulds of the Echini may be sepa- 

 rated by readily-appreciable characters, into what I propose to 

 designate as the true and \he false casts. The false casts are 

 much more abundant than the true, and are distinguished by 

 having upon that portion of their surface which corresponds to 

 the internal face of the ambulacral or perforated plates of the 

 shell, a series of circular and regularly concave pits. Each 

 one of these pits corresponds to an ambulacral perforation ; 

 but it very frequently liappens that the areas occupied by 

 these hollows respectively encroach upon one another, and 

 the whole then become merged into so many deep sulci, ex- 

 tending from the apex of the cast to the base, and indicating 

 the course of the ambulacra. Another condition, much less 

 frequent than the last, but exclusively confined to the false 

 casts, is an abruptly truncate summit, with a surface clearly 

 showing that the deficiency cannot be explained by an acci- 

 dental fracture, but rather suggesting the idea of the siliceous 

 matter having entered at the mouth and vent, (the shell being 



