A RARE BRITISH DIATOM. 39 



B, ch). In the natural habitat of the living diatom the chioma- 

 tophores are brownish, as usual in these plants, but on being 

 gathered the colour quickly changes to green. This peculiarity 

 was mentioned by Ralfs in Pritchard's Infusoria (1861). The 

 ■change of colour may be due to hydrogen sulphide (H2S) develop- 

 ing rapidly in the gathered material and reacting upon the 

 leucocyanin and carotin of the chromatophores. In the process 

 of multiplication by binary division the chromatophores each 

 divide longitudinally into two parts, one part going to each new 

 frustule. In division the two young valves of a double frustule 

 separate first at the centre of the convex dorsal side, and ulti- 

 mately at the ends. In the material supplied by " D. G." no 

 other features of the reproductive process were observable. 



This species is a free marine diatom living amongst the muddy 

 sediment in rock pools within tidal influence. The following is 

 an account by " D. G." of the habitat of his specimens : — " The 

 gathering was made at a little seaside place called Pendine, 

 nine or ten miles east of Tenby on rocks submerged at high water 

 and bare of seaweed, but having little hollows and crevices con- 

 taining a small quantity of mud." 



Among a number of associate species the following interesting 

 iorms also occur in the gathering : Navicula scopulorum, Breb., 

 Navicula {Schizonema)Ta,rcioBisaijna,,Ag.,Sind Toxonidia insignis, 

 Donh. 



Description of Plate 2. 



Amphora inflexa, (Breb.) H. L. Sm. 



A. Valve view x 1,000 diams. ; the dotted lines around this 

 drawing indicate direction of striae. B. Girdle view of a frustule 

 from dorsal side x 1,000 diams. ; r, raphe which from this 

 aspect becomes invisible towards centre and ends of frustule ; 



