286 



/. CULVER HARTZELL 

 TABLE I. — Continued 



Foraminifera, the Coelenterates, except the Sihcispongia, the Echino- 

 dermata, some of the Vermes, the Molluscoidea, and the Mollusca. 



Chitin undergoes more or less akeration. In some cases it is 

 replaced by calcite. 



Silica secreted by organisms is dissolved with comparative 

 ease. It is at times replaced by calcite. The siliceous sponges 

 are very commonly replaced by calcite. If a siliceous organism 

 be found as a siliceous fossil, the original silica has probably been 

 either altered or replaced by silica. 



Carbonate of lime is easily dissolved. It is made use of in two 

 forms by organisms. In the form of calcite it is more durable than 

 in the form of aragonite. This is due to the differences in com- 

 pactibihty, hardness, and specific gravity. Gastropods, many lamel- 

 libranchs, corals, and other organisms composed of aragonite crumble 

 down and pass into calcite, or disappear, while many composed 



