2 74 J- CULVER HARTZELL 



The interior cavities or hollows of a coral may become coated 

 with silica, after which the skeleton may disappear, leaving molds 

 of the interior which will have the appearance of a sponge. If 

 we had a coating which had been deposited upon the inner surface 

 of the shell, it might be easily determined by taking a cast for com- 

 parison with other shells; but in the former case its identification 

 would be extremely difficult, for we might not be able to secure 

 casts from these molds. Even if this could be done, we might still 

 have difficulty in its identification, for we should have only casts 

 of the interior of the cavities or hollows of the original for com- 

 parison. 



DIFFICULTIES ENCOUNTERED 



From the foregoing it is plain that difficulties present themselves 

 even when it is known that the fossil in hand is the original, a mold, 

 or a cast; but the difficulties increase if it be not known what the 

 condition of preservation is. Cases present themselves in which 

 it is a very difficult matter to decide whether the fossil is a cast or 

 a mold; but in the majority of cases this difficulty is obviated by 

 close observation and an understanding of the meaning of casts 

 and molds. 



It is only by a study of casts and molds in their various condi- 

 tions — found as fossils or made in the laboratory — that we may 

 with a certain degree of exactness determine the condition of pres- 

 ervation of a fossil. The internal markings of some forms resemble 

 the external markings of other forms, and it is only in the above 

 way that we may be certain that we are dealing with external or 

 internal markings. 



There is a wide difference between a cast and a mold. Ca ts 

 vary in that some do and some do not show the structure of the organ- 

 ism. Receptaculites oweni. Hall, from the Galena (Lower Silu- 

 rian) at McGregor, Iowa, represents the inner surface of the skele- 

 ton, and is a cast. (See Figs, i and 8.) "In most specimens 

 .... the remains consist of the filling of the intermural space, 

 with casts of the outer surface of the inner wall, the inner surface 

 of the outer wall, and of the connecting tubes.'" This is a calcite 



I Bernard, Principles of Paleontology, Fourteenth Annual Report, New York 

 Geology (1895), pp. 89, 90. 



