30 



many parts of Gteat Britain. Abroad they are often siliceous., 

 or at least contain siliceous crystallizations ; but in Great 

 Britain are mostly calcareous, found in lime-stone rocks 

 and marly places. The shelly part may some of it be the 

 remains of organic structure. The crystallized internal 

 parts of shells and stones afford a curious subject for in- 

 quiry. In the chambers of this nautilus, (for so the living 

 genus is called by Linnaeus, see Gmel. v. I. 3369., the 

 matter of crystallization may have passed through the 

 alveolus, or little hole, r to each partition. In other shells, 

 and in geodes, it must be otherwise. The crystals are 

 rough, and in nearly a regular series from the primitive to 

 the equiaxe. The faces however of the latter are rounded, 

 giving it a lenticular form. They are also somewhat striated, 

 resembling the lenticular crystals of certain spathose iron 

 ores* 



