272 



/. CULVER HARTZELL 



C. The organism may disappear after its cavities or hollows 

 were filled by infiltrating material. In this case we should have 

 only the impression of the interior of the original, and it would be neces- 

 sary to compare this with the interior of skeletons already known. 

 This is at times difficult ; but it is the only alternative. To make this 

 comparison it might be necessary to take an impression, or cast, 

 from the exterior of this filling, which cast would show the markings 

 of the interior of the original. Unless we could find a fossil or a 



Fig. 4. — Michelinia clappi. (After Hall.) 



living form showing the same, or nearly the same, internal markings, 

 we should be baffled in any attempt to adjust it to its zoological 

 position, and hence its identification would remain unsolved until 

 one of these two conditions was satisfied. 



D. The hard parts of an organism may leave only an impression 

 of its exterior in the matrix. It will then be necessary to take an 

 impression, or cast, from this first impression, or mold, and upon 

 the markings shown on this cast will depend the identification of 

 the fossil. 



