10 BRITISH DIATOMACE^S. 



frustule, and gradually decreasing towards the other ; while in Bacil- 

 laria the apposed sides or flanges of the valves throughout the 

 entire length of the frustule are more developed on the exterior than 

 the interior surface of the filament. 



1. Bacillaria paradoxa, Gmel. Length of frustule "0025" to 

 •0042". v.v. 



Ehr. Inf. xvi. Prit. Anim. iii. 166 & 167. Hass. Alg. xciii. 10. Kiitz. 

 Bacill. xxi. 18. Bright. Inf. xv. 



Brackish water. Near Wareham, Sept. 1849. Near Lewes, Aug. 1850, 

 Feb., Aug., Oct. and Dec. 1852, Nov. 1853, Cork Harbour, April and June 

 1855, W.Sm. 



A native of ditches near the sea, or in estuaries subject to marine influ- 

 ences, and not uncommon in such localities. 



Plate XXXII. 279. Supp. PI. LX. 279. 



Genus 32. HIMANTIDIUM, Ehr. 



Frustules linear, arcuate, united into a filament, at first attached, at 

 length free, direct, and slightly arched; valves elongate, linear, 

 arcuate, transversely striated. 



A genus constituted by Ehrenberg, and in which are placed apart 

 from Fragilaria those species in which the frustules are arcuate 

 rather than direct. The separation is judicious, but the allocation of 

 the names unfortunate, as the frustules of the species included under 

 Fragilaria adhere more firmly than those which have been assigned 

 to the present genus. 



The separated frustules of Himantidium bear a close resemblance 

 to those of Eunotia, and differ principally in their more frequent 

 union into filaments. The terminal nodules of Eunotia are probably 

 an appearance arising from the inflection of the valves at the point of 

 junction, and are equally apparent in the present genus ; nor is there 

 any mark to distinguish the valve of the two genera, unless it be in 

 the character of the striae, which in Eunotia are radiate, and in Hi- 

 mantidium parallel. 



There is much difficulty in the discrimination of species in Himan- 

 tidium, the filaments in a growing state appearing precisely alike, 



