Genua of Fossil Uexactinellid Sj^onges, 



19 



fibres into broad niosh-like s))accs ; and we can only distin- 

 ^uisili the site of cacli 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 adde<l 

 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 ajipearance of colour with polarized 

 light. ]iy this we know that the scxradiate spicules of iron 

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

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

 of continuity between the rays of neighbouring sexradiate 

 spicules, wiiich conic to an end abruptly and overlap without 

 passing into one another. 



Fig. 7. 



Fig. 7. fl, botryoidal surface of silicified fibre ; b, c.nsts of spicules in 

 iron-pyntes ; c, radiately crystalline silica of intermesh. 



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

 when the fibre is rejilaced 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 s|)icular rays are sometimes composed of 

 nothing but its sj)herical concretions set in a linear series. 

 The ])yi-ites is not confined to the canals, however^ but forms 

 bacilli, spheiiiles, and granules in the fibre itself, botli when 

 tlie latter retains its original siliceous state and when it is 

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

 intermeslies, taking frequently the form of globidar concretions, 

 which arc covered on the surface with crystalline facets, like 



2* 



