DISTRIBUTION IN SPACE AND TIME 



;4i 



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 palaeoutology 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 :^ — 



"■" CAIN020IC 



MEGAMASTICTORA 



Calcarea Homocoela 



orantiioae 

 Phabetbones 



OlALVTINAE 



Lithoninae 

 MICROMASTICTORA 



Hexactinellioa 



Receptaculitidae 



Hetehactinellioa 



Octactimellida 



TETfiACTlNELLfDA 

 CMOBrSTIDA 

 LlTHISTIDA 



MONAXONIDA 

 CtRATOSA 



> 7 '". 5 



Fig. 123. — Table to indicate disti-ibution 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 



^ Fur fui-thcr details see Zittel, Lclirhuch dcr Pahteontulo(jic, and Felix Bernard, 

 Elements de Palaeontolcgie, 1894. 



VOL. I R 



