VI INTRODUCTION. 



There are many facts of the greatest importance, which 

 the beginner must accept on authority, and as reference 

 to facts of this sort, in an elementary description, for use 

 in the laboratory, could hardly fail to create confusion, 

 such references have been omitted. 



The concrete description of specific forms demands 

 figures of the species described, and as it is important 

 that these figures should show nothing that the beginner 

 cannot himself discover in his specimen, the complicated 

 figures which accompany most monographs were found to 

 be impracticable, and most of the cuts have been made for 

 the purpose, by photographic reproduction of the author's 

 drawings, or of drawings made from nature under his 

 direction. 



Where it has been thought best to reproduce a figure 

 from a monograph, the author has drawn it with a pen, 

 and this drawing has been photo-electrotyped. 



It is hoped that the practicability and significance of 

 the cuts, as guides to dissection and study, will more than 

 compensate for the artistic finish and technical skill which 

 has been lost by the employment of this method of 

 reproduction. 



