Before couclucliug, a word of explanation as to the method adopted 
in my work appears necessary. 
It would have been advisable that all the specimens collected by the 
Bev. W.-B. Clarke should have been figured ; by this means it would liave 
been easy to check my determinations, but circumstances over which I 
have no control not admitting of this, I have confined myself to giving a 
description of them. This description will ho very short in the case of 
species already known, and the identit}^ of which I have no doubt. It will 
consist chiefly in giving the essential characters of each of them, and will 
he accompanied by a reference to tlie Author who first recorded them, to 
him who may have studied it most thoroughly and given the best figures 
of it, and if necessary to him who has described its stratigraphical position 
best. It is therefore to the works of these authors that it will he necessary 
to refer to complete the details I have been compelled to omit, so as not to 
extend beyond limit a work the principal aim of which is to assist in deter- 
mining as exactly as possible the relative age of the different palaeozoic rocks 
of New South Wales. As to the new species, I intend to define them as 
exactly as the state of the specimens will allow me ; I will insist mainly on 
the characters in which they differ or reseml)le their already known congeners, 
so as to render their confusion with these impossible. 
