38 BRITISH DIATOMACEiE. 



and size, but all agreeing in a few general characters. The most 

 important of these is the keeled form of the valves, and the remark- 

 able inequality, in many of the species, between the portions of the 

 valve lying on either side of this pi'ominency. This inequality, or, in 

 other words, this eccentricity of the keel distinguishes Nitzschia from 

 Amphijjrora, in which the keel is also present, while the presence of 

 a keel and its accompanying line or lines of puncta, together with the 

 absence of any form of stipes, separate the present from the genus 

 Synedra, with which several of its species have been united by pre- 

 vious authors. The true character of the valve may be detected by a 

 careful examination of the desiccated frustule, and is easily seen in 

 the figures given in Plates XIII. and XIV. The fractured valve 

 shown in PI. XIV. fig. 118. a, and the sections given in figs. 115. S. 

 and 118. S. will more particularly serve to illustrate the form of valve 

 peculiar to the genus. 



Sect. I. Frustules sigmoid. [Sigmatella, Kiitz.) 



1. Nitzschia sigmoidea, W. Sm. F. linear, sigmoid, truncated; 

 V. linear, suddenly tapering towards the acute extremities ; keel 

 with a single line of puncta; striae 85 in -001". Length -0116" to 

 •0192". v.v. 



/3. F. reflexed; striae distinct, 42 in -001". 



Navicula sigmoidea, Ehr. Inf. xiii. 15. Synedra sigmoidea, Kiitz. Bacill. 

 iv. 37- Nitzschia elongata, Hass. Alg. cii. 12. Sigmatella Nitzschii, 

 Kiitz. Sp. Alg. p. 18. 



Fresh water: very common. Lewes, Dec. 1851, W. Sm. 



ft. Lough Mourne Deposit. Dolgelly Earth. 



Plate XIII. 104. 104 ft. F. V. of single valve of var. ft. Frustule with 

 endochrome. Frontispiece, fig. CIV. 



2. Nitzschia Brebissonii, W. Sm. F. broader and less sigmoid 

 than the last on the F. V. ; V. linear ; extremities attenuate, pro- 

 duced, obtuse; striae 27 in -001". Length -0034" to -0092". v.v. 



Sgnedra Annoricana, Kiitz. Bacill. iv. 34. Sigmatella Brebissonii, Kiitz. Sj). 

 Alg. p. 18. ad specimina quse misit cl. De Brcbissou, Oct. 1852. 



Fresh water. Near Lewes, Oct. and Dec. 1852, W. Sm. 



Siq)l). Plate XXXI. 266. 



