PKEFACE 



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 



