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 which 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 suffer 
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 
