of the Flints of the Upper Chalk. 299 



of the surface of the sponge by fine silt, which caused the pre- 

 servation of form. If such a coating were thus formed, it is ob- 

 vious that the spicula could not escape in the way represented; 

 and the fact is, further, that spicula are not found specially to 

 abound in the mealy coating, as the explanation offered would, 

 at any rate, render necessary. 



I shall presently show, moreover, that Mr. Bowerbank is en- 

 tirely wrong in imagining that the gelatinous substance was 

 poured out of the sponges before they were enveloped in flint. 



On pages 352 and 253 Mr. Bowerbank tells us that the posi- 

 tion of such shells as I described on pages 4 and 5 is '' quite 

 nataraP^ on the outside of sponges. I answer that it is contrary 

 to every law of gravity and to every fact, and that no illustration of 

 similar facts can be produced among recent sponges. He him- 

 self says, just before, that their natural mode of sinking would 

 be to the bottom of the water*. So I say, and beg to refer to 

 the argument on page 5 of my former paper, which has not even 

 been noticed by Mr. Bowerbank. He has shown to me, I must 

 presume, having visited his museum twice for the purpose, all 

 his most characteristic specimens ; and I have certainly not seen 

 in his collection, or elsewhere, a single specimen having the 

 slightest approach in character to the specimens described by me 

 on pages 4 and 5. It is indeed obviously impossible that shells 

 could adhere to any sponge in such positions as in the second- 

 named specimen. The fact of those shells being so found is, on 

 the other hand, a beautiful illustration of the mode of formation 

 of flint suggested by me, namely by afiinity for a nucleus com- 

 bined with molecular attraction. On Mr. BowerbanVs theory, 

 these shells ought, of all places, to be covered with the sponge, 

 sponges especially attaching themselves to solid bodies. But 

 the fact is, that instead of the flint " loosely embracing ^^ (page 

 253) a shell lying on it, as in the cited sponge, the presence of a 

 shell on the surface of a Ventriculite invariably acts as a repellent 

 of the flint. The flint surrounds the shell on all sides, tends to 

 lap a little over in a bulging manner, but does not extend over. 

 Why is this ? Because the soft body had a peculiar afiinity for 

 the liquid silex, the shell had none. Attracted by the soft body, 

 the flint surrounds it — extends as far as the shell ; the molecular 

 attraction causes it to bulge a little over the margin of the shell ; 

 but the combined nuclear affinity being wanting beyond, it ex- 

 tends but little way. This seems veiy simple ; and as it is the 

 test of a true theory or generalization, that every new fact, 

 thoroughly understood, serves only to illustrate it and make it 



* We are not speaking here of such shells as Ostreas and Dianchoras, 

 Avhich affixed themselves by special provisions to surfaces of Ventriculites, 

 &c. ' - 



