iv | PREFACE. 
wise, its state is marked immediately after the letter by which 
it is distinguished ; and if the sexes are known, it is stated 
to be male or female. These particulars are followed by the 
habitat, which is given as particularly as the materials pos- 
sessed by the Museum permit. Those specimens which 
have been presented to the Museum have the name of the 
donor marked immediately after the habitat. 
When there is no such indication, the specimens have been 
either purchased or procured in exchange ; and in this case, 
whenever the place or person from whence. they have been 
received gives authenticity to the specimen, or adds anything 
to their history, they are noted as being from such or such 
a collection. 
_ The various synonyma have been given to the different 
divisions of the class, and to the genera and subgenera and 
species, and a reference made to the works in which they 
have been characterized or described. 
In the adoption of the names for these divisions and for - 
the names of the genera and species, it has been thought 
right to use, whenever it was possible, that which was first 
used for the purpose. As far as regards the specific name, 
there is comparatively little difficulty in the application of 
this simple rule; but ordinal, and especially generic names, 
have been used by different authors in senses so widely dif- 
ferent, and the groups which they are intended to designate 
have been so variously extended and restricted, that it is no 
easy matter to determine, where several names have been 
used, which of them ought to be preferred. 
As every original observer will constantly make use of 
characters which others may have overlooked, or not thought 
of so much importance as further researches have shown to 
belong to it, even when a generic name is used, it will of 
necessity be often employed in a different sense, or with a 
