72 BRITISH DIATOMACE/E. 



envelope of mucus or gelatine. But unfortunately the writers who 

 adopted this division were neither agreed as to the names to be given 

 to these genera, nor the species to be included under each. All the 

 species were at first placed by Agardh in the single genns Schizo- 

 nema ('Systema Algarum/ p. xv, 1824). By Dr. Greville, in the 

 * Scottish Cryptogamic Flora/ 1826 et seq., those with secondary 

 tubes were retained in Schizonema, and the others placed apart in 

 the genus Monema. In his ' Conspectus Criticus Diatomacearum/ 

 1830, Agardh recognizes the distinction suggested by Greville, and 

 places the species with uncompounded fronds in Schizonema, and 

 those with secondary tubes in Micromega ; this arrangement, though 

 unjust to himself, was politely adopted by Greville in the ' British 

 Flora/ Harvey, in his i Manual of British Algae/ reunites the 

 species under the single genus Schizonema ; and Kutzing, in his 

 ' Bacillarien/ 1844, and ' Species Algarum/ 1849, again divides 

 them according to the arrangement in Agardh's ' Conspectus.* 

 None of these methods satisfies Ehrenberg, and he invents a new 

 name, Naunema, ' Infusionsthierchen/ 1838, and again unites all the 

 species under this designation. Here are sufficient elements of con- 

 fusion for future observers. This great diversity of opinion doubtless 

 owes its origin to the variableness and inconstancy of the characters 

 adopted by the writers, who arranged the species under two genera. 

 The presence of only one, or of many files of frustules, is certainly to 

 some extent dependent upon the stage of growth of the specimen 

 examined ; and the appearance of secondary tubes within the general 

 mucus-envelope is more or less apparent in different portions of the 

 same frond, or according as it is examined in the fresh or dry state. 

 A very extensive comparison of specimens leads me to believe, that, 

 in every case where the development of the frond is much advanced, 

 as in the older or basal portions, numerous files of frustules may be 

 discovered ; and as each file, in the progress of self-division, continues 

 to deposit fresh mucus around its frustules, the portion of the enve- 

 lope in their immediate neighbourhood will be gradually consolidated 

 and present the semblance of a tube, which will become more ap- 

 parent on desiccation, as the consolidated mucus shrinks less in 

 drying than the neighbouring portions of the frond. 



Hence has arisen in some fronds rather than others an appearance 



