482 THE PALEON'TOLOGY 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 lamin 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, ¢. 
For the same natural cause, the disunited valves are better calculated to preserve 
the original outline, because they are most likely to le 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. 
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Fig. 38. Illustrating how to obtain a restoration of an obliquely compressed shell. The inner ontline 
represents the specimen as it is now (see fig. 36-c), the outer one a restoration of its original form, S-B. 
plane of sea bottom; d.—p., direction of compressing?force. 
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 
