312 



PRINCIPLES OF ANIMAL BIOLOGY 



histological details, such as the structure of the kidney and of striated 

 muscle are observable in fossils. 



The remnants which we call fossils may represent the original animals 

 in verj' different ways. In some, the original structures are practically 

 unchanged; in others they are completely replaced with minerals; and 

 between these extremes there are various degrees of replacement. The 

 animals, or parts of them, were buried in deposits of silt at the bottom 

 of bodies of water, or under wind-blown material. Bones buried in dry 

 sand in an arid region may be practically the same bones today. But 

 bones buried in silt, which later hardened into layers of rock now far 

 beneath the surface of the land, have been constantly subjected to the 

 action of percolating ground-water. Particle by particle they have 

 been dissolved away, and bit by bit replaced by deposits of minerals 

 carried in solution in the water. This replacement by minerals is known 

 as petrifaction. The petrified object is not converted into minerals, it is 



Fig. 213. — Diagram illustrating molds and casts. Horizontal shading represents sedi- 

 mentary deposits, vertical shading the material subsequently filled in. a, mold of a shell 

 which has been dissolved away by ground water; b, cast formed by subsequent filling in of 

 the cavity of a; c, mold of a shell whose interior was filled with sediment; the shell itself 

 was subsequently removed in solution; d, cast produced by filling the mold represented 

 in c. (From Schuchert's Historical Geology. Courtesy of John Wiley and Sons.) 



merely replaced by minerals. In one fossil this replacement may have 

 gone to completion, the original substance being entirely absent, in 

 another the original material may be interspersed with islands of mineral 

 deposits. In general, the older the fossil the more nearly complete is its 

 petrifaction. 



Molds are produced when the hard substance of a buried animal is 

 dissolved away, but is not replaced by minerals. A cavity in the rock is 

 thus formed, showing the external form of the original. This mold may 

 be subsequently filled, forming a cast; but since the original was not 

 replaced particle by particle the internal structure is not preserved. 

 Figure 213 illustrates the relation of molds and casts to the original object. 

 Casts may also be formed in the cavities of natural objects, such as snail 

 shells; but here also the internal structure of the animal is not shown. 

 Molds and casts are regarded as fossils, though no part of the original 

 object is represented. 



