8 Rev. W. Smitli on the Diatomacese, with descriptions 



8. Pleurosigma ohscurum, n. sp. Valve linear^ slightly attenuated 

 towards the obtuse extremities ; median line not central : co- 

 lour a very pale pink ; strise obscure. Average length of 

 valve y^^ of an inch, greatest breadth of ditto 2 oVu ^^ ^^ ^^^^ 

 [v. v.). 



(3. much smaller, more gradually attenuated. 



In brackish water, Poole Bay, 1849. Lewes, 1851. /3. Poole Bay, 

 covering a sponge. 



No power I have applied has brought the striae into view either 

 in this or the last species, but the colour of the dry valves indi- 

 cates their presence in both cases. 



Plate I. fig. 11. Valve of P. obscurum,', fig. 12. ditto, var. ^3. 

 Section II. Beads opposite, striae transverse and longitudinal, 

 a. Marine. 



9. Pleurosigma Balticum, mihi. Valve linear, straight, suddenly 

 attenuated towards the sigmoid extremities, which are some- 

 what obtuse : colour dark brown. Average length of the valve 

 -^-^ of an inch, greatest breadth of ditto -^J-q of an inch. Strise 

 7^in ^^ ^^ inoh. apart [v. v.) . 



jS. smaller, gradually attenuated. 



•y. much smaller, attenuated throughout ; striae obscure. 



Navicula BalticayYhx., Pritchard's Hist, of Animalcules, 1843, pi. 3. 

 fig. 143 ; Kiitz. Bacill. p. 102. pi. 4. fig. 22 ; Phy. Ger. p. 95 ; 

 Sp. Alg. p. 85. 



Very common, mixed with P. angulatum, in all the localities of that 

 species mentioned above. 



A large and handsome species, distinguished by the parallelism 

 of its sides and the great convexity of its valves : the front view 

 is almost linear. It varies much in size from var. 7, which is 

 probably a very young specimen, and does not exceed -^^q of an 

 inch in length, to others reaching -^-^ of an inch. I cannot ac- 

 count for the absence of a central enlargement of the median line 

 in all the figures of this species given by Ehrenberg and Kiitzing, 

 as in no other of the tribe is this character more plainly discern- 

 ible. 



Plate II. fig. 1. Valve of P. Balticum ; fig. 2. ditto var. jS; fig. 3. ditto 

 var. y; fig. 17. portion of fig. 1 mag. 3200 diameters (Rev. W. Kingsley). 



10. Pleurosigma Strigilis, n. sp. Valve lanceolate, uniformly at- 

 tenuated towards the somewhat acute extremities : colour some- 

 what paler than the last. Average length of valve ^^^ of an 



