CONDITIONS OF FOSSILIZATION ' 277 



Woods ^ applies the term "mold" to (a) the impression, to {h) the 

 material filling the space occupied by the animal. The term "cast" 

 he applies to {a) the material fiUing the space occupied by the 

 organism, and to (h) the material filling the internal cavity or 

 cavities. 



Nicholson and Lydekker^ use the terms "mold" and "cast" 

 interchangeably. 



Lyell^ applies the term "mold" to the matrix in which an impres- 

 sion of the exterior has been made. The term "cast" he applies 



Fig. 8. — ReceplaculUes oweni. Hall. (From specimens in Yale Museum.) 



to (a) the material filling the interior of the organism, and to {h) 

 the material filling the space left by the organism. 



Penning^ uses tlie term "unchanged fossils" for unaltered shells 

 or valves, or those vv^hich have lost only the animal matter. "Re- 

 placed fossils" he applies to the material which has been substi- 

 tuted for original material of the shell. "Internal cast" he uses 

 for "the impression or reversed facsimile of the external form of the 

 organism^ that once filled the empty space" of the shell. The 

 term "external cast" he uses for impressions made by the exterior 

 of the shell, and says that "by taking an artificial cast from the 



1 Woods, Palaeontology, 2d ed., pp. 6, 7. 



2 Nicholson and Lydekker, Manual of Palaeontology, 3d ed., Vol. I, pp. 5, 6. 



3 Lyell, Students' Elements of Geology, 3d ed., pp. 42-46. 



4 Penning, Text-Book of Field-Geology, 2d ed., pp. 208-12, and Fig. 29, p. 211. 



5 The writer's italics. 



