LAMELLIBEANCHIATA. 



481 



Preservation and methods of study. 



Grood 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 flg. 38.) 



The exceedingly diverse results of the pressure, especially in specimens from shale, 

 are most puzzling to the beginner. If a shell happened to stand upon end, its length 



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