INTRODUCTION. 
xxii 
is always attended with difficulty from the causes named, and the influence of 
individual judgment should always be considered. 
In order, therefore, to avoid erroneous conclusions regarding the real charac¬ 
teristics of these fossil shells, it becomes necessary to acquire extensive collec¬ 
tions, not only from one or a few localities, and a single condition of the rock, 
but from numerous and widely separated localities, and from the rock in all its 
phases of composition and conditions of deposition. 
