158 HULL SCIENTIFIC AND FIELD NATURALISTS’ CLUB. 
genre de Pleurosigma,”’ the plates of Diatoms published in the 
‘* Quarterly Journal of Microscopical Science,” and in a minor 
degree from about 30 other works. Some of the figures have 
been drawn specially for this paper, as the species they re- 
present had not been previously satisfactorily delineated. In 
all cases every published figure of the species included has 
been examined, and also the collection of photographs from 
original types by Dr. Maddox and others which are either in 
our possession or were accessible to us. No time has been 
spared in our endeavour to render this list of Diatoms as 
complete as possible, and the plates as good as the space at 
our command would allow. They containa few forms which 
we have decided to omit from the list, the evidence of their 
occurrence in the district being, in our opinion, insufficient. 
These forms are marked 7 in the keys to the plates. 
In revising the nomenclature, we have, in the main, 
followed the classification given by Van Heurck, as his work 
is one of the most recent, and perhaps the most popular and 
and well known to students of the Diatomacee. We have 
however, ventured to diverge from his leading in some few 
particulars. For instance, we considered that the genera 
Pinnularia, included by Van Heurck in Navicula, also Trice- 
ratium and Amphitetras which he includes with Biddulphia, 
were better known under their older names. Many of the 
species recorded as Pinnularia by Norman are however more 
correctly placed among Navicula, and we have accordingly 
referred them to that genus. 
It will be observed that a considerable number of forms 
recorded by Norman as distinct species have, by more or less 
general consent of later writers, been reduced to the status of 
varieties. In this connection we wish it to be clearly under- 
stood that we do not claim the rank of true species, or even 
varieties for every form that we list as such; but merely that 
the specific or varietal name is useful for designating certain 
forms familiar to diatomists. The boundary line between 
species and varieties is too vague and uncertain to admit of 
closer defination than this. 
Not to overload the list with a plethora of synonyms, we 
have used in each case, the name which, in our opinion, is 
the most generally accepted now, and in those cases in which 
that name differs from the ‘one used by Norman, we have 
placed his after it in brackets, ignoring (with a few exceptions) 
all other synonyms. 
We have to acknowledge a few forms in the list on the 
authority of the ‘‘Alga Flora of Yorkshire,” by W. West, 
