26 AN INTRODUCTION TO THE STUDY OF FOSSILS 



many of these shells from the Pleistocene have the calcite pre- 

 served but the aragonite dissolved away. Hence it is seen why 

 shells of brachiopods are better preserved fossils than those of 

 mollusks. 



Calcite may be distinguished from aragonite by the following 

 simple method : powder the substance and boil one minute 

 in dilute cobalt nitrate ; if it is aragonite the precipitate will be 

 of a pink or lilac color ; calcite, even with longer boiling, w^ill 

 remain white (or at times somewhat yellowish from the presence 

 of some organic substance) . 



2. Silica. — Radiolaria (most) ; sponges (Silicispongiae ; this 

 order contains most of the fossil and many of the recent sponges). 



(Silica (SiOo), in the form of quartz, is one of the most stable 

 of minerals, but when secreted by an organism it contains some 

 water, is glassy and isotropic, i.e. is penetrated by light and 

 heat equally in every direction ; in this condition it is dis- 

 solved with comparative ease by percolating waters containing 

 alkali.) 



3. Chitin (C15H26N2O10). — Foraminifera (few); sponges 

 (CeratospongicT) ; Hydrozoa (most, Leptolinae and Grapto- 

 lithida) ; corals (axis of sea-fan, etc.) ; arthropods. (The 

 word chitin is here used in a general sense and does not imply a 

 chemical and structural identity with the true chitin of the 

 Arthropoda.) 



4. Lime phosphate. — Brachiopods (a few, as Lingula, etc.) ; 

 vertebrates (about 40 per cent of the ash of bones is phosphoric 

 acid) . 



5. Cellulose (CeHioOs). — Protozoa (a few); ascidians. 

 Plants. — I. Cellulose (CeHioOs). — All plants from the thal- 



lophytes up, with a few exceptions. 



2. Silica (SiO^). — More or less silica is present in the cell 

 walls of diatoms, equisetes (horsetails), Carex, margins of grass 

 blades, etc. It is especially abundant in the diatoms, the form 

 of the plant being retained by it after the burning away of the 

 organic matter. 



