24 BRITISH DIATOMACE^E. 



have given in Plate XXXV. 298. represents the filament after being burnt, the 

 elliptical outline of the frustules on the F. V. arising from the shrinking of the 

 cell-wall. The present species often secretes during self-division a considerable 

 amount of mucus, which gives to the mass of filaments a gelatinous character ; 

 at other times, this character is altogether absent, and the filament has precisely 

 the appearance of F. virescens, differing from the latter only in its darker and 

 more golden colour, and its habitat, which is exclusively marine. The S. V. 

 of the valve is also nearly that of the species just mentioned, but may be 

 known by its more rounded extremities and its far more delicate striae. All 

 the writers who have described this plant speak of the filaments as attenu- 

 ated ; I can only say that an attentive examination of the species, gathered 

 in various localities and at distant intervals, has failed to satisfy me that any 

 such character is to be found. I have already stated (Introd. vol. i. p. xxvi) 

 that an occasional inequality or slight enlargement in size of the frustules 

 may sometimes be noticed in the filamentous Diatomacese ; but I have never 

 detected anything resembling a gradual increase or diminution in the diameter 

 of the filament, and am disposed to believe that such a condition would be at 

 variance with the laws of growth which prevail throughout the Order, and 

 that the semblance of such a condition in the present species arises from 

 the position of the filament, an oblique direction giving an appearance of 

 attenuation which has no existence in fact. The crenated outline of the 

 margin of the dried filament, as represented by Mr. Ralfs, is also due to the 

 shrinking of the cell-wall. In a fresh and living state the outline of the frus- 

 tule on a F. V. is exactly linear ; but when dried, especially after death or 

 incipient decomposition, the extremities collapse, and the frustules become 

 partially disconnected : hence the appearance which has been described and 

 figured by this careful and trustworthy observer. 



Plate XXXV. 298. 



4. Fragilaria undata, n. sp. Filaments imperfectly tenacious ; frus- 

 tules frequently cohering by their angles ; valve oval, acuminate ; 

 striae 42 in -001". Length of frustule -0006" to -0008". v.v. 



/3. Valve linear, acuminate. Length of frustule "0008" to '00 12". v.v. 

 y. Valve constricted in the centre. Length of frustule '0008 to - 0021". v.v. 

 Var. y. OdontidiumTabellaria. " Sporangial," Greg. M. J. vol. ii. pl.iv.22. 



Fresh water. Var. y. Mull Deposit. (The three forms in River Mortes, 

 Mont Dore, elev. 4066 ft., June 1854, W. Sm. Near Christiania, Dr. Arnott.) 



Living specimens which I collected in the French locality above mentioned 

 have enabled me to assign the present species to the genus Frayilaria, with 

 which its mode of growth, filamentous character, and delicately striated valves 

 ally it more closely than with any other genus. 



Supp. Plate LX. ?>77 . 



