ZOOLOGICAL AND BOTANICAL ASSOCIATION. 95 



the Desmidiaceae the distinctive characters of the genera are usually 

 sufficiently well marked and decided ; while, with two exceptions of 

 genera, each containing only one species, they are formed of often nu- 

 merous species, embodying and presenting, in, generally, an obvious 

 manner, the characters of the genus under which they fall. I think, 

 then, it is repugnant to a proper classification to unite in any genus, the 

 species of which in an evident manner, by the possession of common 

 characteristics, form a natural assemblage, one or more other species 

 (although as a matter of course there must be minor points of affinity) 

 which positive and negative characters of form and habit, it may be 

 slight, but decided, and not held in common with the true species of the 

 genus, exclude from being therein embraced. 



A belief in the correctness of the opinion I here endeavour to express 

 has emboldened me to the step which in this paper I now venture to 

 take, in an attempt to form a new genus, and which I do with much 

 diffidence ; for, being wholly without the guidance of any of our masters 

 in the study of these minute Algae, or the benefit of their opinion, I 

 would put forward the following, rather as suggestive than conclusive, 

 more in the hope of, perhaps, ultimately eliciting information and cor- 

 rection, than as indisputably settling the position of the forms presently 

 to be described. 



It will, perhaps, be more convenient that I first proceed to describe 

 the, new forms which I have met with, one of which belongs to the genus 

 Sphaerozosma, another to Staurastrum, while the remaining two fall 

 under the proposed new genus ; and then to enumerate the species as de- 

 scribed in Ralfs' monograph supplementary to my former catalogue, in- 

 cluding therewith those species now for the first time noticed. 



Before drawing attention to the new forms, however, it has been 

 suggested to me, in deference to those of this Association (as well as to 

 those who may hereafter favour this paper with a perusal) who, not 

 having immediate access to Ralfs', or the " Micrographic Dictionary,' ' 

 &c., or not having directed their attention to the organisms themselves, 

 may be comparatively unacquainted with the Desmidiaceae, or kindred 

 minute Algae, that a short account of their nature and position in the 

 vegetable kingdom may not be uninteresting ; and while endeavouring 

 to do so I have to apologize to those to whom this sketch will, I fear, 

 prove tiresome and dry, from being to them so familiar, because so devoid 

 of novelty or originality. I hope, then, that those who have made these 



