PREFACE 
ONE of the greatest difficulties experienced by all who undertake a 
work of this nature, not professing to be an exhaustive treatise 
on the subject with which it deals, is to determine the amount 
of detail desirable to be introduced to meet the requirements of 
the ordinary student, without rendering it too bulky or costly 
for general use. The experience of those who endeavour to profit 
by the book can alone decide how far the authors have succeeded 
in this respect. It will be observed that in many instances certain 
better-known or more interesting members of the class have been 
described at considerable length, while it has been necessary to 
treat others with much greater brevity. . . 
With regard to the references to the literature of the various 
groups treated of, it has been the endeavour of the authors to 
make a selection of such memoirs and works as are likely to prove 
most valuable to the student for the amount of original informa- 
tion which they contain, and more especially of those giving 
full bibliographical data up to the time of their publication, the 
repetition of which has been considered unnecessary. 
In a few instances new generic terms have been introduced to 
