158 THE CANADIAN NATURALIST. [Vol. X. 



The crystuiliue form of aragonite may be considered as an 

 abnormal form of carbonate of lime, aod Mr. Sorbj shows that 

 under various circumstances, it is easily resolved into the more 

 stable form of calcite. whilst the carbonate, in the crystalline 

 form of calcite, cannot be changed by any known process into 

 that of aragouite. These two minerals form the principal con- 

 stituents of the tests of shells — in some classes the aragonite 

 being prevsent, in others the calcite, and again in others the 

 inner layer may be of aragonite and the outer of calcite, or vice 

 versa. 



Mr. Sorby gives the following clai>sification of the mineral com- 

 position of the different orders of shells: 



(a) Crustacea. — The mineral matter of crustaceans consists of calcite 

 hardened on the surface with phosphate of lime. 



(i) Cephalopoda — These shells are made up of aragonite together 

 with a small amount of phosphate of lime. 



(c) Gasleropoda. — In most of these genera the shell is wholly made 

 up of aragonite, but in some the outer layer consists of calcite. 



(c?) Lamellihranchiata. — In many species of this group the tests are 

 composed wholly of aragonite, in some entirely of calcite, whilst other 

 shells have their inner layer of one material and the outer of the 

 other. 



{e) Brachiopodaj are composed wholly of calcite. 



(y) Echinodermata. — Here the mineral matter is calcite. 



(y) Polyzoa are composed of various mixtures of both minerals. 



(A) Hydroida and true corals are made up of aragonite — the former 

 class having a small quantity of phosphate of lime. 



{€) Foramini/era are probably composed of calcite. 



The removal of the organic matter holding the particles of 

 the shell together disturbs the stability of the structure, and 

 not only causes it to crumble by the disintegration along the 

 lines between the different minute crystals, but also hastens a. 

 subsequent re-arrangement of the molecules into larger and less 

 constrained crystals. Especiiliy is this the case with fragments- 

 of aragonite which soon take the form of calcite, as is shown by 

 the experiment of Mr. Sorby, where powdered coral (aragonite) 

 kept for only a few weeks in water began to change into tlie con- 

 dition of calcite. Moreover, this is not only an experimental 

 test under favorable circumstances, but it is found that the modern 

 limestones now forming about some of the West Indian Islands, 

 have in places entirely lost or are losing the natural forms of 

 the organic fragments of which they are composed. Again, the 



