Nodules of the Trimmingham Chalk. 443 



or epizoic life upon others ; several grew crowded together on 

 the same object of support, just as at the present day one may 

 find no less than seven different species of sponge growing 

 together on one small fragment of Lophohelia not an inch 

 square *. With death and the dissolution of the organism 

 the spicules were set free from the different adjacent sponges, 

 and, falling into the same deposit, naturally mingled together ; 

 movements of the surrounding sea-water may very well have 

 taken place, and would serve to render the mixture of the 

 spicules more complete. In this way would be produced 

 a layer of chalky ooze crammed with sponge-spicules of all 

 sorts and sizes. Such sponges as possessed skeletons coherent 

 enough to maintain their general form after death would be 

 covered up and filled in with this mixture of ooze and spicules, 

 and, undergoing silicification, would furnish us with instances 

 of fossil sponges presenting a well-preserved form externally 

 and a curious mixture of spicules within. For some sugges- 

 tive observations on this subject Dr. Wallich's paper on " the 

 Natural History of the Cretaceous Flints " may be con- 

 sulted f. 



Excepting that sponges do not periodically shed their 

 spicules like leaves and spores, the explanation we have just 

 suggested bears a striking resemblance to the " growth-in- 

 place " theory of our coal beds. In the coal, as in the flints, 

 the structure of the constituents has generally been almost 

 entirely obliterated, yet some few of the leaves or spicules, 

 as the case may be, are occasionally found in an admirable state 

 of preservation; and just as a Slgillaria every now and again 

 remains a solitary survivor of a whole forest, so now and then 

 a whole sponge is to be found preserved out of a host of 

 associates now vanished or turned to flint. 



(iii) The solution of the Sjyicules. — From the preceding 

 paragraphs it is clear that solution of sponge-spicules has been 

 of very common occurrence. A summary of the evidence in 

 proof of this, however, may not be out of place here. Thus : — 

 1, fossil sponge-spicules are frequently eroded externally and 

 their axial canals enlarged internally ; 2, all flesh-spicules, 

 necessarily once pi*esent, have entirely disappeared ; 3, in 

 many chalk-flints Ventriculite and Lithistid skeletons occur, 

 perfectly preserved as to form, but not as solid network, 

 merely as empty casts ; 4, the skeletons of many fossil 

 sponges have exchanged a siliceous for a calcareous com- 

 position. 



As to the reality of the alleged solution there can be no 



* Carter, Ann. & Mag. Nat. Hist. ser. 4, vol. xii. pi. i. figs. 1, 2. 

 t Quart. Journ. Geol. Soc. vol. xxxvi. p. 68. 



31* 



