14 AN INTRODUCTION TO THE STUDY OF FOSSILS 



York, and fish in the Newark beds (upper Triassic) of New 

 Jersey and Massachusetts. 



Objects indicating the former presence of organisms. — 



Molds. — When a leaf falls upon soft mud, it will impress 

 there its form and surface markings ; but the sun may quickly 



Fig. 3. — Diagram to explain molds, external and internal, and casts. The 

 shell is the common ark, Area transversa from Long Island Sound. A, external 

 surface of right valve ; B, the concave external mold of same valve ; C, interior 

 view of right valve ; D, internal mold of same valve ; E, section through entire 

 shell from hinge-line {h) forward, the shell being embedded in and filled with 

 sediment (w) ; F, external ornamentation impressed upon the internal orna- 

 mentation through the settling of the sediment after the chemical removal of the 

 shell (as from Fig. E) by water percolating through the strata, ext. m., external 

 mold ; h., hinge-line ; int. m., internal mold ; m., matrix, or sediment ; miis., muscle 

 scars; r. v., right valve; sh., shell (or, if shell is removed and this cavity is filled 

 by some foreign substance, this substance is termed a cast of the valve, since it 

 is formed between molds). (X^.) 



curl its edges so that it will be blown away by the wind. If 

 this impression or mold be covered by sediment, the imprint 

 is preserved without the leaf itself being inclosed. The mold 

 is thus shaped into the form and surface character of the original. 

 If a shell, such as a clam or snail, be covered with sediment, it will 

 impress its external ornamentation upon the surrounding material 

 (Fig. 3, B). The motion of the water aided later by the weight 



