4 CALCAREOUS SKELETONS. 



dition in which the carbonate of lime exists. In some 

 animals it occurs as aragonite, in others as calcite. Of 

 these two minerals, aragonite is the harder and heavier,, 

 its specific gravity being 2'93, whilst that of calcite is. 

 only 277. Aragonite is rhombic and optically biaxial, 

 calcite is hexagonal and uniaxial. Fossil calcite shells- 

 (e.g. Pecten opercularis) are translucent, their surface is. 

 compact, but their interior porous ; on the other hand the 

 aragonite shells (e.g. Pectunculus glycimeris) are opaque, 

 and have a chalky appearance and a compact structure 

 throughout. When a shell of each kind, having the same 

 size and weight, is suspended in water containing carbonic 

 acid, it is found that the one composed of aragonite loses 

 in the same time a much greater amount of matter than 

 the one of calcite. And further, the calcite shell remains 

 firm much longer than the aragonite, the latter soon 

 coming to have the consistency of kaolin, and is then 

 easily broken up and carried away by a stream of water. 

 This difference, however, does not appear to be due directly 

 to mineral composition, for Messrs Cornish and Kendall 

 found that when ordinary crystalline calcite and aragonite 

 were powdered and placed in carbonic acid solutions of 

 the same strength, the aragonite was not acted on more 

 rapidly than the calcite, and the same result was obtained 

 with powdered fossil shells. From all these considerations, 

 it is not surprising to find that in some strata the 

 aragonite skeletons have entirely disappeared, whereas 

 those formed of calcite remain. This will obviously be 

 most likely to occur where the bed is pervious and where 

 there is a flow of carbonated water. In the case of the 

 mussel, Mytilus edulis, in which the inner layer of the 

 shell is formed of aragonite and the outer of calcite, 

 Dr Sorby found that specimens from the raised beach at 



