384 DESCRIPTION OF [Poli/ffasfrica. 



flexible, not fragile ; frustiiles (testules, Ehr.) rectangular, plane, 

 not striated, scarcely siliceous. 



In appearance, this genus comes very near to Fragilaria, with 

 which it is united by most writers, but its habit is so very different, 

 that I am inclined, with Agardh, to keep them distinct. In Fragi- 

 laria the filaments are very fragile ; the species do not adhere well 

 to paper ; the frustules are siliceous and glass-like, and may be sub- 

 jected to a red heat, without any other alteration than the destruction 

 of the colouring matter ; and at each end are two, more or less dis- 

 tinct, pellucid puncta. 



In Grammonema there is scarcely any silica, in which important 

 character it differs from most of the Diatomem, the fflaments are not 

 fragile, but highly mucous, adhei-ing firmly to paper or glass, and, 

 when dried, appearing like a mere stain ; the application of nitric 

 acid, or of a red heat, destroys their form, and I can perceive no 

 puncta at the end of the frustules. The filaments are elongated, 

 ribbon-like, and composed of numerous fi'ustules, which are longer 

 than broad. (Ralfs in Ann. Nat. Hist., vol. 13, 1844, p. 457.) 



Geammonema Jurgmsii = Fragilaria aurea (Hooker). — Filaments 

 attenuated, yeUowish-brown ; frustules three to eight times longer 

 than broad ; slightly separated at the angles (P. 24, f. 24, 25.) 



Var. fh ) Biatomoides. — Filaments green when dried, and elongated, 

 giving a feathery appearance to the plants to which they are attached, 

 very mucous, flexible, gradually attenuated ; fi-ustules under the 

 microscope nearly colourless ; slightly attenuated at both ends, and 

 hence disconnected at their angles ; and as the ends are often also 

 somewhat rounded, the margins of the filaments have a crenate 

 appearance. In a mass, both varieties are dark brown, but much 

 paler if separated in the water. In the first variety the colour is 

 but little altered in drying. 



Genus H ALTON YX. — Lorica bivalve, orbicular, not concatenated; 

 BUi-face of disc rayed ; number of rays definite, not starting from the 

 umbilicus ; no internal septa. 



It resembles Actinocyclus, except in its umbilicus not being radiate, 

 or, in other words, the central ocellus is wanting. In like manner, 

 Comnodificus differs from Sgwboloplwra in its non-radiant umbilicus, 

 wliich is a simple void space. 



