84 BRITISH DIATOMACE^E. 



slight dorsal elevations ; stria? obscure. Length *0007". Breadth 

 of valve -0002". v.s. 



Ehr. in Kiitz. Bacill. xxix. 59 ; Microg. xxxv. A. ii. 10. Grev. Ann. 2nd ser. 

 vol. 15. pi. ix. 3. 



Fresh water. With the last species, and probably only an advanced 

 state of it. 



4. Cymbella ventricosa, Kiitz. Dorsal (larger) segment of V. regu- 

 larly convex, basal linear, margin plane ; striae 30 in '001". Length 

 •0006" to -0011". Breadth of V. -00025". v.v. 



Kiitz. Bacill. vi. 16. 



Fresh water : frequent. Cockshut Stream, Lewes, May 1853, Sec, W. Sm. 

 Derbyshire, May 1853, Br. Redfern. Queen's Park, Edinburgh, April 1854, 

 Br. Greville. Lough Leven, Aug. 1854, Br. Gregory. Haverfordwest, Aug. 

 1855, Mr. Okeden. (Langogne, in the Cevennes, elevation 4000 feet, June 

 1854, W.Sm.) 



5. Cymbella Helvetica, W. Sm. 



Var. j3. Extremities obtuse, or rounded, v.v. 



Fresh water: very frequent. Near Menai Bridge, July 1853, W.Sm. 



Probably the most usual form of this species : hardly to be distinguished from 

 detached frustules of Cocconema cymbiforme, as figured in ' Synopsis/ xxviii. 220. 



6. Cymbella Scotica, W. Sm. 



Var. /3. "With obtuse or rounded extremities ; striae 42 in 'OOl". v.v. 



Fresh water: often intermixed with C. Scotica, a. Braemar, Aug. 1854, 

 Br. Balfour, very plentifully. 



The number of striae, in Vol. i. p. 18, is wrongly stated at 32 in -001". 

 This species comes very near C. delicatula, Kiitz. Sp. Alg. p. 59. 



7. Cymbella sequalis, W. Sm. Valve nearly direct, and symmetri- 

 cal, slightly curved at the produced and obtuse extremities ; striae 

 30 in -001". Length -0013" to -0015". Breadth of V. -0003". v.s. 



W. Sm. in Grev. Ann. 2nd ser. vol. 15. pi. ix. 4. 



Fresh water. Braemar, Aug. 1854, Br. Balfour. New Forest, Hamp- 

 shire, Sept. 1854, W.Sm. 



8. Cymbella lunata, W. Sm. Frustule lunate ; dorsal segment of 



