228 DESCRIPTION OF \_Poli/ffastrica. 



lobcd, warted or spinous. The internal matter is of a herbaceous 

 green coloui-, and starch vesicles abound in the mature cell. They 

 couple and form either orbicular or quadrate seed-like bodies, and are 

 remarkable for the resistance which they oppose to decomposition. 



*' In all these respects they diifcr from Diatomacca. In the latter, 

 each fnistule consists of thi-ee pieces, one central and ring-like, or 

 continuous all round ; and the others lateral. The division is com- 

 pleted by the fonnation of new portions within the enlai'ged central 

 piece, which then falls off, or else by a new septum arising at the 

 centre ; but I believe that, in every case, the separation commences 

 internally before it extends to the covering. Their coverings, with 

 very few exceptions, are siliceous, withstand the actions of fire and 

 acids, and may be broken but not bent ; the frustules arc often 

 rectangular in form, are never warted, and scarcely ever spinous. 

 Their internal matter is usually brown when recent ; and, although 

 some species are greenish, or become green after they are gathered, 

 none are of a truly herbaceous colour. Their vesicles bear some 

 resemblance to those in the Besmidieee, but they are of a yellower 

 colour, and no starch has been detected in them. Some of them 

 have been seen to conjugate, but their sporangia are elongated and in 

 pairs, and the internal matter is similar to that of the frustules." 

 (British Desmidiese, page 20.) 



In the following pages we have so far departed from Ehrenberg's 

 aiTangement as to include the Closterina with the Desmidiece, in 

 accordance mth the now universally received views of their affini- 

 ties. Moreover, we have excluded the appended and doubtful group, 

 Acineta, according to the opinion of Ehrenberg liimself, as expressed 

 subsequently to the publication of his systematic work. 



The particular details of the organization of the Desmidiaeea and 

 Navicuhcea, are entered into at the commencement of each of those 

 sections ; those relating to the Ecliinellea, presenting nothing special, 

 are represented in the account of the Navicidacea. Indeed, in our 

 opinion, the section EcMnelha should be merged, as is generally done 

 in that of the Naviculacea, but, as we profess generally to foUow the 

 system of Ehrenberg, we have thought it right to retain it. 



