4 Scientific Proceedings, Royal Dublin Society. 



following calculation : — The average of from five to seven is 6. It 

 takes 8*18 sponges to produce one flint; and supposing these to grow 

 one after another, end on, it will take 8-18 x 6 = 49-08 years for the 

 formation of the material of a flint of the size of a saleable bath 

 sponge ; and a bed of such flints would be formed in the same 

 time, which is considerably less than the lifetime of a healthy man. 

 It is very probably true that the rate of growth is not the same 

 for bath sponges and silicious sponges ; but a number of considera- 

 tions show that if the silicious grow less rapidly than the others, 

 yet, as far as the formation of silica is concerned, this may be 

 compensated for in a variety of ways. We have supposed the 

 successive sponges to grow in a linear series, one after the other, 

 and have taken no account of the swarms of young sponges which 

 they produce at each breeding season ; no doubt most of these are 

 prematurely destroyed, since every sponge must have sufficient 

 space about it for, so to speak, breathing purposes ; but the ex- 

 terminated young forms leave their quota of silica behind, since 

 they commence its secretion even from the planula stage. Again, 

 the total quantity of silica found at any one time in an adult 

 sponge is not necessarily all that it has" produced during its life- 

 time ; for, as I have elsewhere shown, some silicious sponges, if not 

 all, are constantly extricating some of the spicules they have 

 secreted ; and these dead spicules, discharged around them over the 

 sea floor, may bear a not inconsiderable ratio to those remaining in 

 the sponge. In one case (Cyclonium neptuni) they accumulate 

 within certain cavities of the interior of the sponge, completely 

 filling them up, so that they look as if stuffed with cotton wool : 

 the quantity of spicules so preserved must, I should imagine, 

 amount to at least one-fifth of the total quantity present in the 

 sponge; but these are only what, by an accident of structural 

 character, are preserved and can be seen. How many others have 

 been extruded at the surface, fallen on the sea floor, and left no 

 trace of their previous connection with the sponge behind, we do 

 not know, and have no means of knowing. But it is a very 

 suggestive fact that at one of the stations from which the 

 " Challenger " obtained sponges in great numbers (Station 149, 

 off Kerguelen), the mud of the sea floor from which the sponges 

 were dredged is crammed full of sponge spicules ; and though 

 the majority of these may have been derived from dead sponges, yet 



