54 6 [Proc. B.N.F.C., 



(b) Fresh-water Fossils. — In this case we have to deal with 

 shallow waters containing little mineral matter in 

 solution. The conditions are obviously different from 

 those that prevail in ocean sediments where we have 

 large quantities of dissolved solids and greater depths. 

 The only specimens of silicified fresh-water fossils that I 

 have examined appeared to indicate silicification by 

 hydro-thermal agencies long after sedimentation. 



(c) Crustacea. — The shells of this group are quite different 

 in chemical composition from those of the mollusks and 

 corals which we find beekised. They are composed 

 chiefly of chitin, which is a very insoluble cellulose 

 derivative containing a much smaller proportion of 

 calcareous matter than the former. The mineral part is 

 frequently the less soluble phosphate to a great extent. 

 A chitinous phosphatic shell is thus clearly less liable to 

 suffer siliceous replacement in the presence of shells 

 composed of carbonate of lime. 



(d) Cephalopoda (with few exceptions). — This is the most 

 highly organised group of the Mollusca, and the horny 

 organic matter present in the shells appears to possess 

 quite a different nature from the gelatinous residue 

 obtained by dissolving the shells of the lower mollusks 

 in acid. The only examples of beekite on this group I 

 have seen in the Cretaceous beds of Co. Antrim were 

 two specimens of the calcite guard of Belemnitella very 

 slightly beekised. 



On the ' whole it is a remarkable fact that the calcareous 

 structures chiefly affected and replaced by beekite are those which 

 possess a netted or spicular structure (such as corals and sponges) 

 or a honeycombed structure (such mollusks as the Lamelli- 

 branchia), and which leave a gelatinous colloid residue (described 

 by Carpenter as "animal glue") when digested in acid. This 

 substance is probably allied in composition to the gelatinous 



