TYPES OF FOSSILS. 3 



by a continental formation, i. e., a lake, river, or wind deposited 

 formation. Examples of such more or less equivalent marine and 

 non-marine formations are the Chemung and Catskill, the Pierre 

 and Belly River, and the marine Tertiaries of the Atlantic and 

 Gulf Coasts and the non-marine Tertiaries of the plains and Great 

 Basin region. * 



Types of Fossils. — Four types of fossils may be recognized, 

 namely: (1) Actual remains and their impressions; (2) trails and 

 tracks made by organisms in transit, and burrows; (3) artificial 

 structures; (4) coprolites. 



I. Actual Remains. — Generally only the hard parts of animals 

 will be preserved but occasionally the fleshy parts also remain. 

 Examples of this are the mammoths and other animals preserved 

 in Siberian ice and the Tertiary insects of the amber found on the 

 shores of the Baltic. The hard parts most readily preserved are 

 the bones of vertebrates, the plates and spines of the external 

 armor of fishes and reptiles, the exoskeleton of Crustacea, the 

 plated armor or test of echinoderms, the shells of mollusks and 

 brachiopods, the " coral " structures of Bryozoa, corals, and hydro- 

 corallines, and the shells of Protozoa. The horny covering of hy- 

 droids is also more resistant than other soft animal tissue and is 

 commonly preserved in the form of carbonaceous films. Plant 

 tissue is more readily preserved than animal tissue ; especially is 

 this true of wood, which is commonly preserved by being vari- 

 ously impregnated or replaced. 



Molds and Casts. — Frequently the actual remains of the organ- 

 ism decays or is dissolved, leaving only a mold behind ; when the 

 structure was hollow, as in the case of shells, both external and in- 

 ternal molds (the latter often called casts) remain. Occasionally, 

 by the infiltration of mineral matter into the mold, a cast of the ori- 

 ginal shell is produced. In the study of molds artificial casts must 

 frequently be made.f 



2. Trails, Tracks and Burrows. — Foot-prints of vertebrates on 

 soft mud or sand and trails of worms, mollusks, or Crustacea are often 

 preserved. Here no part of the animal remains, nor is this the only 



*See the table of formations in the appendix (published with the last install- 

 ment). 



I For processes of making these see appendix. 



