170 Mr. G. H. K. Thwaites on the Diatomacese ; 



Found in small quantity in a rapid stream of fresh water on 

 the filaments of Vaucheria, roots of grass, &c, at Crew's Hole 

 near Bristol, in December last. 



Schizonema subcoharens, n. sp. PL XII. fig. G 1 ( \ ), 2 (^), 3- 

 7 (— ). Cselomatibus in massam amorpham subcohserentibus, 

 valde mucosis, ramosis, navicularum ssepe multas singulas 

 series continentibus : naviculis late truncatis, versus apices 

 subito angustatis, striatis. 



Evidently closely allied to Schizonema ? mucosum, Kiitz. Bacill. 

 115. tab. 26. fig. 9, but differs from it in having the frustules 

 striated and towards the apices suddenly narrowed. Tufts of the 

 plant from a quarter to half an inch or more high ; filaments very 

 mucous and tenacious, and each containing from one to several 

 single rows of frustules, which are continued without interruption 

 into the branches. 



For the opportunity of figuring and describing this interesting 

 species I am indebted to the kindness of the Rev. W. Smith, 

 F.L.S., who found it in June last in Wareham " North River," 

 densely spreading in a spongy stratum over the clayey bank and 

 bottom of the stream. 



The sporangia of this species, fig. G 6, are produced by the 

 conjugation of a pair of frustules outside the filaments ; but spo- 

 rangial frustules are frequently found in a filament intermixed 

 with ordinary frustules, from which they differ only in size : 

 from this and from what has been observed by the writer in 

 other species, it would appear that the frustules have a tendency 

 to arrange themselves into linear series, and that subsequently a 

 mucous sheath is developed around them. 



Schizonema subcoharens would appear to belong to Agardh's 

 genus " Micromega " : it is difficult however to see the advantage 

 of creating a new genus from characters derived from the mu- 

 cous sheath only, and which characters really may be present in 

 some species without being clearly evident. The so-called " sper- 

 matid " of Micromega, now that the true sporangia have been 

 discovered, require further examination : a somewhat similar ap- 

 pearance to what is figured by Kiitzing is sometimes evidently 

 due to minute zoophytes in an immature state. 



Schizonema vulgare, n. sp. PL XII. fig. H 1 (f ), 2-5 (*). Na- 

 viculis la3vibus, lanceolatis, versus apices subito angustatis. 

 Hob. in aqua dulci. 



Var. a. rivulorum. Fig. H 1-4. Cselomatibus distinctis, ra- 

 mosis : naviculis subacutis. 



