132 



FIELD GEOLOGY. 



thus revealing the impression or reversed facsimile of 

 the external form of the organism that once filled the 

 empty space. 



Suppose, for instance, we are dealing with the remains 

 of a bivalve shell, such as a Trigonia in the figure, 



Fig. 20a. Trigonia clavellata, Kimmeridge Clay. 

 Cast and Impression. 



Tve find that the cast exhibits two raised ledges occupy- 

 ing the position of the muscular impressions, connected 

 by a slightly raised ridge, corresponding to the pallial 

 line which marks the attachment of the animal's mantle ; 

 the interspaces between the teeth will also appear, and 

 sometimes concentric or radiating ridges answering to 

 those of the shell itself. In the same way the internal 

 structure of any hollow body, whether coral, univalve 

 shell, or^echinoderm, is faithfully represented by its cast, 

 and where opportunities for comparison exist, its genus 

 and even its species may in many cases be ascertained. 



