of British species of the genus Pleurosigma. 7 



5. Pleurosigma strigosum, n. sp. Valve lanceolate, flexure slight, 

 extremities obtuse: colour straw. Average length of the 

 valve fo of an inch, greatest breadth of ditto ^ of an inch. 

 Striae 7¥o -<jo of an incn a P art (* 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 t }q of an inch, 

 greatest breadth of ditto ¥ | F of an inch. Stria? jj^oo °f an 

 inch apart {v. v.). 



ft. Valve narrow lauceolate, slightly quadrangular. 

 y. Valve much smaller, quadrangular. 



Var. /3. Navicula angulata, Quekett on the Microscope, 1848, p. 438. 

 pi. 8. figs. 4, 5, 6 & 7. Nav. Mstuarii, Kutz. 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. Johnson, 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 striae renders its desiccated valve 

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

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

 usually requires a higher power, and the youngerfrustules 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. (3. 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. j3 ; fig. 9. 

 valve of ditto var. y. 



7. Pleurosigma distortion, n. sp. Valve lanceolate, abruptly bent 

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

 obscure. Average length of valve T |^ of an inch, greatest" 

 breadth of ditto TJ l w of an inch (v. v.). 



With P. angulatum, coast of Sussex, 1850. 



Probably the young of some other species. 



Plate I. fig. 10. Valve of P. tKstortum. 



