of British species of the genus Pleurosigma. 7 



5. Pleurosigma strigosum J n.^^. Valve lanceolate, flexure slight, 

 extremities obtuse : colour straw. Average length of the 

 valve -^Q of an inch, greatest breadth of ditto ^i^ of an inch. 

 Striae ^^^^^ of an inch apart [v. v.). 



Rye, Mr. Jenner, 1849. Coast of Sussex, 1850. Hull, Mr. R. 

 Harrison ! 



Its stouter habit, darker colour, and more distant striae di- 

 stinguish this species from P. elongatum, 



Plate I. fig. 6. Valve of P. strigosum. 



6. Pleurosigma angulatum, mihi. Valve broadly lanceolate or 

 quadrangular, extremities acute, flexure moderate : colour a 

 bright chestnut. Average length of valve yf ^ of an inch, 

 greatest breadth of ditto ^|^ of an inch. Strise ^j^-^q of an 

 inch apart [v. v.), 



/3. Valve narrow lanceolate, slightly quadrangular, 

 y. Valve much smaller, quadrangular. 



Var. /5. Navicula angulata, Queketton the Microscope, 1848, p. 438. 

 pi. 8. figs. 4, 5, 6 & 7. Nav. ^stuarii, Kiitz. Sp. Alg. 1849, p. 490. 



On the mud of tidal harbours ; very common. Poole Bay ; Bel- 

 fast Bay, Liverpool ; Coast of Sussex, &c. ; Coast of Lancashire, 

 Chr. Johnsoriy Esq. ! Rye, Mr. Jenner ! Hull, Mr. R. Harrison ! 

 Norfolk, Thos. Brightwell, Esq. ! 



This species has long been a favourite test with the micro- 

 scopist, and the fineness of its strise renders its desiccated valve 

 very suitable for such a purpose. The strise may be detected 

 with a power of 400 diameters, but to resolve them into beads 

 usually requires a higher power, and the younger frustules demand 

 the best glasses and most careful manipulation for their resolu- 

 tion. The quadrangular form of this species is always more or 

 less present, and seems to distinguish it from all its allies ; but 

 for this circumstance var. yS. might be confounded with the last 

 species ; it is however quite distinct. 



Plate I. fig, 7. Valve of P. angulatum ; fig. 8. valve of ditto var. /3 ; fig. 9. 

 valve of ditto var. y. 



7. Pleurosigma dis tor turn, n. sp. Valve lanceolate, abruptly bent 

 towards the obtuse extremities : colour a very pale pink ; strise 

 obscure. Average length of valve -^^-q of an inch, greatest 

 breadth of ditto yjV(J of an inch (v. v.). 



With P. angulatum, coast of Sussex, 1850. 



Probably the young of some other species. 



Plate L fig. 10. Valve of P. distortum. 



