I. . \MKI.I. mil \N- IMAtA. 



I cast- of the interior are aUo to be met with in shaly rocks, indeed, most 

 excellent ones when the shales are arenaceous. In soft shales, like those of the 

 1 in. -iMn.it i 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 

 palfo/oic formations when compared with their frequent occurrence in Carbonif- 

 erous deposits. 



The mo-t f.norable method of preservation, so far as Lower Silurian material is 

 concerned, i- that in which the originally calcareous shell is more or less completely 

 replaced l>y -ili.-a. S;ich specimens are rare in the Northwest, but common in the 

 ><>lii| limestones of the Trenton in Tennessee and Kentucky, and in the Hlack Kiver 

 limestone of Canada. Beautiful specimens of this kind are to be found weathered 

 out. or I. lock- 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 

 \\hether or not the material, as it lies before us, has retained its original form. 



rtion through pressure in the rock matrix is a most fruitful source of error 

 a ii. I one that even the greatest experience cannot entirely negative. It is evident 

 that the -ofter and, consequently, the more yielding the character of the matrix, the 

 greater the degree of the distortion. It is least in limestones and dolomites ami 

 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. Ilt,utr.,tia distortion of *hlU through premurr side of a ftpeclrnen of ModMoptta 



modiolari* Conrad, th- hutht of which has been reduced, ai shown In mitliti-. to lew than half what It wan 

 originally. 6. a bell of the aame gpecics greatly comprmatd teogthwls. -h<-ll <>f an undeacribed 



pecle* of OKMcmya. from <>lii<>. lilustratlng the effect of prcj*urc on HhelU .m-upjrfnK an otll'iue po- 

 in thenhaleu. The line *-b indicates the plane of the (traUaod tea l<>tt"iii. (See OK. 38.) 



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



are most puzzling to the beginner. If a shell happened to stan-l uion eml. its lei 

 -a i 



