Naviculacea.] infusorial animalcules. 503 



narrowing near the apices, and striated. Closely allied to S. (?) nm- 

 cosum (Kiitz.), but differs from it in having the frustules striated ; and 

 towai'ds the apices suddenly narrowed. Tufts of tlie 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. The sporangia 

 of this species are produced by the conjugation of a pair of frustules, 

 outside the filaments; but sporangial frustules are frequently found in 

 a filament intermixed with ordinaiy frustules, from which they differ 

 only in size. This species appears to belong to Agardh's genus 

 Micromega, but it is difiicult to see the advantage of creating a new 

 genus from characters derived from the mucous sheath only, and 

 which characters really may be present in some species, without being 

 cleai-ly evident. The s,o-cq]1c& spermatia of Micromega, now that the 

 true sporangia have been discovered, require frirther examination; 

 a somewhat similar appearance to what is figured by Kiitzing, is 

 evidently due to miuute Zoopytes in an immatiu'c state. 



ScHizoNEMA tJ?«/^yflre (Thwaites.) — Naviculae smooth, lanceolate, 

 suddenly contracted near the apices. Habitat in fresh-water. 



Var. [a.) rivulorum. — Tubules distinctly ramose; naviculse sub- 

 acute. 



Var. {h.) laaisfre. — Tubules mucoiis, simple (or sparingly branched ;) 

 naviculae more broadly truncate than in j^receding variet}^ Monema 

 lacmtre (Agardh.) 



Var. (c.) effusum. — Tubules distinct, diffused through a gelatinous 

 sti-atum ; naviculce as in var. {a.) 



Although this is perhaps the commonest species of all the Schizo- 

 nemafa, since it occurs, during the spring, in almost every ditch and 

 running stream, yet it does not appear to have hitherto been de- 

 scribed, unless the Monema lacustre (Ag.) should prove to be one of 

 its forms. This species is most abundant in shallow streams, covering 

 stones, &c., with a dark brown gelatinous coating, but in which a linear 

 arrangement of the frustules may frequently be detected. When the 

 plant occurs in deeper water, the ordinary Schizonema filaments make 

 their appearance, which are much branched when growing in rapid 

 streams, but in stiU water are simple or nearly so. In the last 

 named fonn of the species, perhaps the Monema lacustre, there is also 



