288 /. CULVER HARTZELL 



Gastropoda. — Aragonite. Some are composed of aragonite and 

 calcite. 



Cephalopoda. — Mainly aragonite. Nautilus pompilius has cal- 

 cite for its inner layer and septum, instead of aragonite as hereto- 

 fore reported. 



Crustacea. — Mainly calcite. 



REPLACING MINERALS 



The hard parts of invertebrate organisms are composed of more 

 or less soluble mineral matter, and are often replaced by other min- 

 erals which fill the cavities left by the hard parts. There may be 

 molecular replacement as the original gradually disappears, or the 

 cavity may be filled by precipitation after the original has entirely 

 disappeared. Chemical reaction may take place, producing new 

 minerals as the elements in the original unite with the elements in 

 the matrix, or elements brought in due to the porosity of the imbed- 

 ding material. 



The imbedding material always contains minerals that are easily 

 dissolved under such conditions as heat, pressure, and moisture, 

 and they may be deposited separately or in combination. The 

 predominating mineral is apt to be found forming molds or casts 

 of the lost parts. 



In calcareous shales we find calcite casts. In siliceous lime- 

 stones we find siliceous casts. In ferruginous formations we find 

 siderite, pyrite, limonite, etc., casts and molds. In galena-bearing 

 formations we find casts composed of that sulphide. These illus- 

 trations might be extended; but they sufiice to show how the char- 

 acter of a formation affects an original skeletal part in its preservation. 



The most common replacing minerals are calcite, pyrite, silica, 

 limonite, sphalerite, vivianite, barite, malachite, siderite, and hem- 

 atite. The list of replacing minerals is quite large, thirty-five being 

 the number. Others undoubtedly occur, and sooner or later will 

 be added to our present list. Under Table II is given the replacing 

 minerals found, and their symbols, Dana's system being followed 

 in their classification. 



In the paper to follow will be given a table showing the mineral 

 composition of the more closely related living and fossil forms studied. 



