of the Chalk. 183 



mens of the same species having different degrees of thickness, 

 and there would be no distinct under-skin such as I have de- 

 scribed. It is quite clear that these polypidoms are true polypi- 

 doms, the living skeletons of living polyp-masses, and not the 

 remains of dead polyps piled one tier above another. 



The careful examination of a suite of the best-preserved speci- 

 mens in my possession at length revealed to me a character 

 which I immediately felt must prove of the utmost importance 

 in the present question. I observed, in casts of some of the 

 most perfectly preserved specimens, minute perforations extend- 

 ing from the surface which had been in apposition with the 

 fossils inwards into the matrix of the cast. Further observation 

 satisfied me that this was no accidental appearance, but that it 

 owed its existence to the original of the fossil itself. I found 

 traces of the same appearance also in the casts of specimens 

 preserved in a fine state in flint in those cases in which the 

 remains of the Ventriculite were found in the condition of an 

 open network only, the phenomena of which have been above 

 explained. 



In order to arrive at any just conclusion from these ob- 

 servations, it was necessary to apply here again all the consider- 

 ations which have been already noticed, as to the nature of the 

 two very different matrices in which the Ventriculidse are found. 

 Those considerations showed it to have been a necessary con- 

 dition that the substance, whatever it was, which gave ori- 

 gin to these perforations, was neither of the merely soft and 

 readily decomposable nature of the substance which filled the 

 central tissue of the polypidom, nor yet either composed of a mi- 

 nutely fibrous membrane like the fibrous parts of that polypidom, 

 or being a single projecting fibre overspread with soft or mucila- 

 ginous matter, like a sponge. Were it the former, its place 

 would be most usually filled up, in chalk specimens, by chalk, 

 as the intermediate spaces of the squares have become filled up. 

 Were it either of the latter, we should sometimes find in the flint 

 traces of the fibres hermetically sealed, and in flint and chalk we 

 should find the tubular incrustments and other conditions 

 already stated as marking the former presence of the central 

 fibre. But none of such conditions has been ever found. It follows 

 that a substance differing from either formerly filled these minute 

 perforations, a substance little if at all less durable than the 

 skeleton fibre, far more durable than the mere soft intermediate 

 substance. Such a substance precisely answers to the terms 

 under which a true epidermis is properly described, which is not 

 fibrous in texture but pretty nearly homogeneous, and so strong 

 as to have been described by a well-known modern author as 



