FOSSIL SPONGES, ETC. 11 



zoological objects, as everybody acquainted with 

 the recent species Eiiplectella aspergillum ("Venus' 

 flower-basket ") will readily allow. The accompany- 

 ing wood-cuts show the appearances of recent and 

 fossil sponge-spicules under the microscope. 



The siliceous sponges are again sub-divided into 

 several groups, named respectively Monactinellidcs, 

 Tetractinellida, Hexactinellida (that is, " one, four, and 

 six-radiated spicules"), and Lithistidcs. We know 

 nothing of fossil sponges except the mineralized 

 spicules they have left behind, and these have been 

 frequently much changed since the sponges were 

 alive. Limy spicules are often dissolved away, and 

 perhaps left no trace behind them, unless the hol- 

 lows they had left have been filled with flint. But 

 there are numerous cases where even flint spicules 

 have been dissolved, and where lime has replaced 

 silica. Until a few years ago, it would have been 

 much questioned whether the siliceous spicules of 

 sponges could be replaced by any other mineral 

 matter ; but there is now little doubt that organic silica 

 can be removed, although geologists can only record 

 the observed fact without explaining it. Some 

 siliceous spicules have been replaced by iron pyrites, 

 in the Lower Cambrian rocks of North Wales. 



Nevertheless, fossil sponges possessing a siliceous 

 or flinty skeleton, are likely to be much more numer- 

 ously preserved than those sponges whose skeletons 

 were composed of a less endurable mineral. Hence 



