with desa'iptions of new genera and species. 171 



Var. /3. lacustre. Fig. H 5. Cselomatibus mucosis, simplicibus 

 (aut parce ramosis ?) : naviculis latius truncatis quam in varie- 

 tate prsecedenti. 



Monema lacustre, Agardh ? 



Var. 7. effusum. Cselomatibus indistinctis, in stratum gelatino 



sum effusis : naviculis ut in varietate a. 



Although this is perhaps the commonest species of all the 

 Schizonemata, since it occurs during the spring in almost every 

 ditch and running stream, yet it does not appear hitherto to have 

 been described, unless the Monema lacustre of Agardh should 

 })rove to be one of its forms. The species is most abundant in 

 shallow streams, covering stones, &c. with a dark brown gelati- 

 nous 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 

 when occurring in still water, or where there is only a slight cur- 

 rent, are simple or nearly so. In the last-named form of the 

 species, which may possibly be the Monema lacustre, Ag., there 

 is also a slight difference in the form of the frustules, which are 

 rather shorter compared with their width, and more truncated at 

 their extremities. The frustules of all three varieties are of a 

 lanceolate form, suddenly narrowed near the apices. 



Schizonema neglectum, n. sp. PI. XII. fig. J 1 {■^), 2-4 (?f). 



Cselomatibus ramosis, mucosis : naviculis lanceolatis, delica- 



tule striatis. 



The filaments of this species, which are branched, especially 

 towards the base, easily escape detection owing to particles of 

 sand and other substances adhering to their tenacious surface 

 and being with difficulty removed from it. It is therefore next 

 1.0 impossible to get good examples of this species, and hence the 

 reason why it has hitherto escaped the observation of botanists. 

 The frustules are lanceolate and very delicately striated, and are 

 very like those of Schizonema floccosum, Kiitz., which has been 

 found by Dr. Dickie near Aberdeen ; but in that species they ai'e 

 not striated, and are moreover included in a gelatinous sheath of 

 much greater thickness than that of Schizonema neglectum. 



Occurring amongst other Diatomacea; from fresh or slightly 

 ijrackish water near Bristol. 



Dickicia Danseii, n. sp. PI. XII. fig. K 1 (^), 2-4 (f ). Frons 

 gclatinosa, indefinita, mammillosa : naviculis ovalibus, striatis. 

 Tlie frustules of this species are siliceous and of an oval form, 



