BRITISH DIATOMACE/E. 23 



Wells, Dec. 1842 and Mar. 1843, Mr. Jenner. Aberdeenshire, Dr. Dickie. 

 Lanarkshire, October 1855, Dr. Arnott. Fell End, Lancashire, May 1851, 

 Mr. Johnson. Near Haverfordwest, April 1854, Mr. Okeden. Near Ulver- 

 stone, Aug. 1854, Miss E.Hodgson. Lough Mourne Deposit. Marl, Co. Down. 

 Premnay Peat and Mull Deposit. Var. /3. Ashdown Forest, March 1844, 

 Mr. Jenner. (Falaise, 31. De Brebisson. Genolhac in the Cevennes, elev. 

 3600 feet. Pic de Sancy, elev. 6000 feet, June 1854, W. Sm.) 



This species, first collected in Britain by Mr. Jenner, and described by 

 Mr. Ralfs, is correctly outlined by Ehrenberg under the name of F. pectinalis ; 

 yet, as he describes his species as striated, and as it is certain that with the 

 defective instruments he employed he could not have detected the transverse 

 striae on the true F. virescens, we must conclude that he examined specimens 

 intermixed with Himantidium pectinate, and remarking the notable striae on 

 the frustides of the latter, concluded that they belonged to the present 

 species. 



The ordinary condition of F. virescens is undoubtedly that of a filament of 

 considerable tenacity, the frustules, like those of Odontidium mutabile, part- 

 ing in the centre, rather than separating by the adjacent surfaces ; but in cer- 

 tain stages of growth, or under peculiar circumstances of development, this 

 character appears to be lost, and partial dismemberment takes its place, the 

 frustules still cohering with considerable tenacity, but only by their alternate 

 angles, as in other genera hereafter to be described. In this condition our 

 present plant has been transferred by Hassall and Kutzing to Diatoma, its 

 alliance with which genus we regard as accidental, the normal condition of the 

 filament being characterized by a tenacity wholly unknown in Diatoma. 



Plate XXXV. 297. Var. /3. Plate XXXV. 297/'. 



3. Fragilaria stria tula, Lyng. Valve linear, somewhat attenuated 

 towards the obtuse extremities; striae 64 in '001". Length of 

 frustule -0009" to -0016". v.v. 



Lyng. Tent, lxiii. A. Grammonema Jurgensii, Ag. Consp. p. 63. Ralfs, 

 Ann. vol. 13. pi. xiv. 5. Fragilaria aurea, Car. Grev. B. F. p. 403. 

 Harv. Man. p. 197. ad specim. authen. in Herb. Grev. Fragilaria dia- 

 tomoides, Grev. B. F. p. 403? Harv. Man. p. 198? Grammatonema 

 striatulum, Kiitz. Sp. Alg. p. 187. 



Marine. Coast of Sussex, Sept. 1850, April and May 18*52, W. Sm. 

 Appin, Capt. Carmichael. Frith of Clyde, April 1853, Dr. Landsborough. 



Though less firmly siliceous than many others of the filamentous Diato- 

 maceae, the present species undoubtedly belongs to this Order, and cannot be 

 located with the Desmidiece, as has been done by Professor Kutzing in the 

 ' Species iUgarum.' The behaviour of the frustules under nitric acid is similar 

 to that of the other filamentous species, and the cell-wall resists the action of 

 fire ; the frustules, though losing somewhat of their rigidity under the action 

 of the latter agent, still retain sufficient of their form and character to show 

 that silex must be largely combined with their substance. The figure we 



