482 



THE PALEONTOLOGY OF MINNESOTA. 



[Preservation and methods of study. 



will be greatly reduced; if upon its base, the hight; if upon its side, the thickness. 

 When these positions were in no wise oblique, the beginner may fail entirely to 

 notice the distortion, which, when their position in the strata is oblique to the plane 

 of deposit, will be more or less clearly obvious to him because of the unsymmetrical 

 forms of the two valves. A careful examination, however, will reveal, at any rate 

 on specimens that have not been much weathered, certain fine parallel lines on the 

 sides of the crushed shell. These lines are coincident with and probably produced by 

 the deposit laminae of the matrix, and an experienced student may, with their aid, at 

 once determine the direction and perhaps the amount of the reduction of the par- 

 ticular dimension affected. It is to be remembered that the pressure under which 

 the fossils suffer acts, except in comparatively rare instances, in a vertical direction 

 only. Complete shells are generally compressed more or less obliquely, for the simple 

 reason that after the death of the animal the natural position of the shell, with 

 respect to the plane of the sea bottom, must be approximately as shown in fig. 37, c. 

 For the same natural cause, the disunited valves are better calculated to preserve 

 the original outline, because they are most likely to lie upon their inner edges, the 

 latter being, therefore, at right angles to the direction of the pressure; in which 

 case, under ordinary circumstances, the only dimension that can be altered is the 

 thickness, this being reduced according to the amount of compression sustained by 

 the surrounding rock. 



Fig. 38. Illustrating how to obtain a restoration of an obliquely compressed shell. The inner ontline 

 represents the specimen as it is now (see flg. 36-c), the outer one a restoration of its original form, S-B. 

 plane of sea bottom; d.-p., direction of compressingiforce. 



In making the restoration shown in fig. 38, only the two regions or points 

 a and b can be assumed as having retained their positions on the original boundary, 

 because there alone the outline of the shell coincides with the direction of the com- 

 pressing force. The only effect the latter could have had upon them was to increase 

 their convexity and to press them down slightly beneath their original positions. 

 On all other points, however, the effect was a reduction in the convexity of the 



