Genus of Fossil Hexactinellid Sponges. 



19 



fibres into broad mesh-like spaces ; and we can only distin- 

 guish the site of each by the fact that the botryoidal surface 

 always presents its bosses towards the meshes and away from 

 the interior of the fibre ; to which distinction may be added 

 another, which consists in the fact that the silica deposited 

 via the fibre is never fibrous like that deposited^ outside, but 

 gives merely a mottled appearance of colour with polarized 

 light. By this we know that the sexradiate spicules of iron 

 pyrites are truly inside the fibre, as we should expect, and not 

 outside, as they appear to be. Here, again, we find a want 

 of continuity between the rays of neighbouring sexradiate 

 spicules, which come to an end abruptly and overlap without 

 passing into one another. 



Fig. 7. 



Fig. 7. a, botryoidal surface of silicified fibre ; h, casts of spicules in 

 iron-pyrites ; e, radiately crystalline silica of intermesh. 



Iron Pyrites. — This, as we have seen, fills the central canals 

 when the fibre is replaced by crystalline silica ; but it does so 

 as well when the fibre retains its original state and when it is 

 converted into carbonate of lime. It is always granular — so 

 much so, that fine spicular rays are sometimes composed of 

 nothing but its spherical concretions set in a linear series. 

 The pyrites is not confined to the canals, however^ but forms 

 bacilli, spherules, and granules in the fibre itself, both when 

 the latter retains its original siliceous state and when it is 

 wholly clianged into calcite. It is, moreover, found in the 

 intermeshes, taking frequently the form of globular concj-etions, 

 which are covered on the sui'face with crystalline facets, like 



2* 



