14 



of material. It would sometimes happen that a very small mass 

 of the fluid would approach the outside only of a large Ventri- 

 culite. In such case none could reach the inside, and we should 

 find, and do, that it is external only. 



It may at first sight be thought that there would be hollows 

 in chalk specimens similar to those in flint, and that these would 

 be as subject as the latter to have any inclosed fibre incrusted 

 with calcedony. A little reflection will however satisfy the careful 

 reader that this could not be the case. The mould of the indi- 

 viduals inclosed in flint was perfectly solid. Though sulphuretted 

 hydrogen were evolved, which it necessarily would be, from the 

 decomposing body, the encasement in that solid matrix would 

 generally prevent the so free deposit of sulphuret of iron as would 

 take place in the open chalk. Consequently the remaining fibre 

 would offer more affinity for the gaseous silex than would such 

 fibre remaining in the open chalk and more highly charged with 

 sulphuret of iron. And the fact is, that specimens preserved in 

 the chalk exhibit a much larger proportion of sulphuret of iron 

 than those preserved in flint, frequently so much that the spe- 

 cimen, after a short exposure to the air, becomes a mere mass of 

 oxide of iron, and all structure is un distinguishable. In other 

 cases it is less so, and I have in fact found, in several chalk spe- 

 cimens in which the presence of iron is the least marked, that the 

 deposit of crystallized calcedony* pure and without the presence 

 of a particle of flint on either external or internal surface has 

 taken place to a small extent, but in a most exquisite manner. 

 It is clear however that, in general, when any part of the soft 

 substance of a body encased in chalk decomposed, its place was 

 soon filled up with particles of chalk, which in its then scarcely 

 hardened state were readily carried in. The firmer fibre would, 

 thus inclosed, endure for a much longer period, probably indeed 

 until the chalk had become comparatively dry, on which event 

 its absorption, where little sulphuret of iron was present, would 

 follow and the space be left vacant. The instances of calcedonized 

 fibre in chalk specimens are then quite as frequent, and to just 

 such an amount in individual cases, as might be anticipated, while 

 the frequent excellent preservation of the forms of the Ventri- 

 culidse in the chalk is also explained. 



A piece of dead fibrous skeleton in the chalk would evolve but 

 little sulphuretted hydrogen, consequently would induce the de- 

 posit of little sulphuret of iron. It follows that, when buried in 

 the chalk mud, it would be at once closely encased. The hard- 

 ened chalk would, on the absorption of all the components of the 

 fibre, also leave the places of those fibres vacant. On cutting open 

 these, as well as on cutting open specimens encased with their 

 soft parts and whose phenomena it is above attempted to ex- 

 * Not flint : it is important to remember this. 



