278 /. CULVER HARTZELL 



external impression" we obtain "an accurate representation of 

 the pre-existing shell." 



Williams^ is perplexing in his use of the terms "mold" and 

 "cast." He says: 



Thus a fossil .... may consist of the shell now removed, in which case 

 it may be the reverse or cavity over the exterior of the shell, or ... . similar 

 impressions of the inner surface; or the cavity may be again filled with detrital 

 matter, forming a cast of either the inner or outer form of the shell or object 

 fossilized; in the former case it would be called a mold; in the latter, a cast. 



Schuchert^ appHes the term "mold" to impressions of the exte- 

 rior and speaks of the mold as "preserving the exterior form and 

 ornamentation" of the shell. He is ambiguous in his use of the 

 term "cast" for he may be referring to a matrix which contains 

 concave impressions (impressions of the exterior of a valve) or 

 convex impressions (impressions of the interior of a valve), or to 

 the material which replaces a valve. 



DEFINITIONS OF " MOLDS " AND " CASTS " FOLLOV^ED 



The definitions followed in this paper are the following: A 

 mold is "a form or model pattern of a particular shape, used in 

 determining the shape of something in a molten, plastic, or other- 

 wise yielding, state." "In founding, a mold is the form into which 

 a fused metal is run to obtain a cast. "^ 



The mold determines the shape of the material put in or upon it, 

 and this material, when removed, will be an exact duplicate of the 

 object from which the mold was made. This removable material 

 is termed a cast. The depressions in the original object will appear 

 as protuberances in the mold, and the protuberances as depressions. 

 The cast will show the depressions and protuberances as they appear 

 in the original. 



The surface of the original object upon or around which the 

 mold is made may be either convex or concave. If it be concave, 

 the mold will be convex, and vice versa. A mold with a convex 

 surface is called by some authors a "cast." If the skeletal part 



1 Williams, Geological Biology (1895), p. 79. 



2 Schuchert, "Directions for Collecting Fossils," Part k. Bulletin No. 39, U. S. 

 N. M., p. 13. 



3 Century Dictionary, "Mold." 



