LAMELLIBRANCHIATA. 481 
Preservation and methods of study. 
Good casts of the interior are also to be met with in shaly rocks, indeed, most 
excellent ones when the shales are arenaceous. In soft shales, like those of the 
Cincinnati group of Ohio, they are generally preserved as partial moulds of the 
exterior. The approximately unaltered shell is to be counted as rare in lower 
paleozoic formations when compared with their frequent occurrence in Carbonif- 
erous deposits. 
The most favorable method of preservation, so far as Lower Silurian material is 
concerned, is that in which the originally calcareous shell is more or less completely 
replaced by silica. Such specimens are rare in the Northwest, but common in the 
solid limestones of the Trenton in Tennessee and Kentucky, and in the Black River 
limestone of Canada. Beautiful specimens of this kind are to be found weathered 
out, or blocks of the limestone may be treated with dilute acid with the same result. 
The first essential in the study of fossil Lamellibranchiata is to determine 
whether or not the material, as it lies before us, has retained its original form. 
Distortion through pressure in the rock matrix is a most fruitful source of error 
and one that even the greatest experience cannot entirely negative. It is evident 
that the softer and, consequently, the more yielding the character of the matrix, the 
greater the degree of the distortion. It is least in limestones and dolomites and 
greatest in shales and slates. The direction of the distortion depends upon the 
position occupied by the shell with respect to the bed planes of the enclosing rock. 
Fig. 37. Illustrating distortion of shells through pressure. a, right side of a specimen of Modiolopsis 
modiolaris Conrad, the hight of which has been reduced, as Shown in outline, to less than half what it was 
originally. b, a shell of the same species greatly compressed lengthwise. c, the shell of an undescribed 
species of Cuneamya, from Ohio, illustrating the effect of pressure on shells occupying an oblique position 
in the shales. The line s-b indicates the plane of the strata and sea bottom. (See fig. 38.) 
The exceedingly diverse results of the pressure, especially in specimens from shale, 
are most puzzling to the beginner. Ifa shell happened to stand upon end, its length 
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