INTRODUCTION. xxi 



In the gradual dessication and hardening of the soft muds in which the shell 

 was originally imbedded, extreme changes in form are sometimes wrought, as 

 is more distinctly shown where two valves held in connection by the liga- 

 ment, one has been imbedded horizontally, retaining essentially its original 

 form, while the other, imbedded vertically, or with its basal margin downward, 

 is narrowed by the vertical pressure to a degree which leaves it quite unlike in 

 form to the corresponding attached valve, as has been shown in a few illustra- 

 tions in this volume. Where such influences are exerted upon single discon- 

 nected valves we have no means of knowing the degree of distortion produced 

 except by comparison with other valves recognized as of the same species. 

 Shells Avhich in their original condition may have been convex or even gib- 

 bous, have become flattened by the pressure, especially in the thinly laminated 

 beds ; and unless found under more favorable conditions, their true proportions 

 may not become known. A slightly oblique pressure may change the position of 

 the beaks relative to the anterior or posterior extremities 'of the shell without 

 distorting the general form. The general outline of the shell may also sufier 

 from very slight movements of the solidifying matrix. 



The surface markings may become more or less obliterated and sometimes 

 modified by a covering of very fine sand which adheres to the shell. The 

 sharpness of the surface striae is frequently subdued or nearly obliterated by 

 maceration, or from the presence of iron pyrites, where decomposition leaves 

 the resulting acid to attack the prominent portions of the surface and thus 

 modify or destroy the exterior characters. 



Through the influence of such modifying conditions it may often happen 

 that characters which are regarded as of importance in the living shell may be 

 obscured or obliterated during the process of fossilization. 



Considering the effect of all these influences, it often requires great care to 

 determine what are really specific distinctions, and what modifications may be 

 due to extraneous causes. With a small number of individuals, derived from 

 one or two localities, it may not always be possible to make the proper discrim- 

 ination. The true determination of the limits of species in this class of fossils 



