46 BRITISH DIATOMACE.E. 



Genus 19. NAVICULA, Bonj. 



Frustules free, oblong or lanceolate ; valves convex, with a median 

 longitudinal line and nodules at centre and extremities, striated ; 

 strise resolvable into circular dots. 



In the present work, the genus Navicula of Kiitzing is divided into 

 three ; viz. Navicula, Pmnularia, and Pleurosigma. Stauroneis has 

 already been placed apart both by Ehrenberg and Kiitzing. 



The four genera are, I think, sufficiently distinguished from each 

 other ; Navicula, by the delicacy of its striae and their moniliform 

 character ; Pinnularia, by the confluent nature of the cellular structure 

 of its epiderm, which gives to its striae the appearance of distinct ribs 

 or costae ; Stauroneis, by the absence of a central nodule, which is 

 supplied by a band of silex (Stauros) crossing the valve at right 

 angles to the median line ; and Pleurosigma, by the characteristic 

 curve of its beautiful frustules. 



In a few cases we meet with the semblance of a Stauros in the 

 genus Pinnularia ; but in these instances a closer examination will 

 show that this appearance arises from the interruption of the costae 

 merely, and not from the dilatation of the central nodule, which is 

 still found unchanged. 



Sect. I. Valves lanceolate. 



1. Navicula rhomboides, Ehr. V. nearly quadrangular; striae very 

 faint, parallel, 85 in -001". Length -0022" to -€037". v.v. 



Ehr. in Kiitz. Bacill. xxviii. 45, xxx. 44, sic Prof. Kiitzing in lit. 1851. 



Fresh water. Boggy pools, Sept. 1849, W. Sm. Lancashire, Mr. Johnson, 

 Aug. 1851. Ben M-^ Dhui, at an elevation of 3000 feet, Mr. P. Grant. 



Plate XVI. 129. 



2. Navicula lanceolata, Kiitz. V. lanceolate or elliptical-lanceolate ; 

 striae parallel, '44 in 00 1". Length -0012" to -0020". v.v. 



Kiitz. Bacill. xxrai. 38. xxx. 48 ? 



Freshwater. Newhaven, Sussex, Dec. 1852, TF. Sui. Dolgelly Earth. 

 Supp. Plate XXXI. 2/2. 



