BRITISH DIATOMACE^E. 39 



but as our Synoptical arrangement is designed to aid in the identifi- 

 cation of species by a statement of the most obvious characters, it 

 appeared necessary to place Diatoma, with its zigzag filament, near 

 others presenting a like notable peculiarity. 



No difficulty can arise in the discrimination of the species of the 

 present genus. Fragilaria virescens is the only form which might 

 possibly be assigned to Diatoma ; but this error will be avoided by 

 remembering that the zigzag, which is rare and accidental in F. vires- 

 cens, is a constant and distinctive feature in all species of the present 

 genus. 



1. Diatoma vulgare, Bory. Valve elliptical, suddenly attenuate 

 towards the obtuse extremities, costate ; costae pervious, somewhat 

 irregular, about 18 in -001". Length of frustule -0007" to -0021". 

 Breadth of V. -0003" to -0005". v.v. 



/3. "Valve linear, extremities gradually and slightly attenuate. Length of 

 frustule -0018" to -003". v.v. 



Ralfs, Ann. vol. 11. pi. ii. 8. Kiitz. Bacill. xvii. 15. Raben. Siissw. Diat. 

 pi. ii. Hass. Alg. xciv. 1. Bacillaria vulgaris, Ehr. Inf. xv. 2. Var. /3. 

 Diatoma tenue, Ag. ad specim. authen. in herb. Grev. 



Fresh water. "Wareham, Dorsetshire, May 1849 ; Lewes, Sussex, April 

 and Sept. 1852 ; Glasheen, near Cork, April 1855, W. Sm. Ardrossan, April 



1853, Dr. Landsborough. Lough Leven, Oct. 1854, Dr. Arnott. Var. /3. 

 Plumpton, Sussex, April 1852 ; River Lee, near Cork, April 1855, W. Sm. 

 Pentland Hills, April 1821, Dr. Greville. (Canal du Midi, and Canal de 

 Crapone, South of France. /3. Fountain in Hotel des Colonies, Marseilles, 

 May 1854, W. Sm.) 



Plate XL. 309. Var. /3. Plate XL. 309 /3. 



2. Diatoma grande, n. sp. Valve linear, constricted near the 

 rounded extremities, costate ; costae pervious, 24 in •001". Length 

 of frustule -0013" to -0038". Breadth of valve -00025" to -0003". v.v. 



Fresh water. River Lune, Lancashire, April 1848, Mr. G. Smith. River 

 Shannon and River Corrib, July 1853; Lough Derg, July 1855, W. Sm. 

 Lough Neagh, Dr. Dickie. Lough Leven, May 1854, Dr. Gregory. Allan 

 Water, Stirling, May 1855, Dr. Greville. (River Sorgues, Vaucluse, May 



1854, W. &».) 



I was at first disposed to assign this form to D. Ehrenbergii, Kiitz. ; but as 

 Professor Kiitzing describes and figures the latter with a central inflation of 

 the valve, while the present is linear, and as authentic specimens of D. Ehren- 



