DISTRIBUTION IN SPACE AND TIME 



241 



the final station of a sponge ; for six days is not an excessive 

 interval to allow for the larval period of at any rate some species. 

 Distribution in Time. — All that space permits us to say on 

 the palaeontology of sponges has been said under the headings of 

 the respective classes. We can here merely refer to the chrono- 

 logical table shown in Fig. 123 :^ — - 



MEGAMASTICTORA 

 Calcarea Homocoela 



CaLCAREA hetcrocoela 

 Svcettidae 



Grantiidae 

 Phahetrones 



OlALVTINAE 

 LlTHONINAE 



MICROMASTICTORA 

 Hexactineluoa 



Receptacuutidae 

 Heteractinellioa 



OCTACriNELLIDA 



Tethactinellida 

 Choristioa 



LlTMISTIDA 



monaxonida 

 Ceratosa 



Pig. 123. — Table to indicate distribution of Sponges in time. 



Flints. — The ultimate source of all the silica in the sea and 

 fresh-water areas is to be found in the decomposition of igneous 

 rocks such as granite. The quantity of silica present in solution 

 in sea water is exceedingly small, amounting to about one-and- 

 a-half parts in 100,000 ; it certainly is not much more in 

 average fresh water. This is no doubt due to its extraction by 

 diatoms, which begin to extract it almost as soon as it is set free 

 from the parent rock. It is from this small quantity that the 

 siliceous sponges derive the supply from which they form their 

 spicules. Hence it would appear that for the formation of one 



1 For further details see Zittel, Zehrbuch der Palaeontologie, and Felix Bernard, 

 EUments de Palaeontologie, 1894. 



VOL. I I' 



