BRITISH DIATOMACE^. 61 



9. Stauroneis punctata, Kiltz. V. elliptical, extremities produced ; 

 stauros linear, central; striae radiate, punctate, 27 in '001". v.s. 



Kiitz. Bacill. xxi. 9. 



Fresh water. Lough Mourne and Peterhead Deposits. Premnay Peat. 

 Ben M'^ Dhui, Mr. P. Grant. Cantyre Peat. (San Fiore, Italy.) 



Plate XIX. 189. 



10. Stauroneis pulchella, n. sp. F. V. oblong, constricted at the 

 centre ; V. elliptical ; stauros central, dilated towards the margin ; 

 striae very distinct, 30 in "OOl", punctate ; puncta hexagonal. 

 Length -0048" to -0096". v.v. 



/3. Dry valve purplish, somewhat acuminate. 



Marine. Coast of Sussex, Aug. 1850, and Poole Bay, Sept. 1851, W. Sm. 

 /3. Torbay, July 1846. Coast of Sussex, Aug. 1850, W. Sm. Near Ipswich, 

 Mr. Hodffson, Aug. 1851. Havwich., Mr. Shadbolt. (Peruvian Guano.) 



Plate XIX. 194. 6. outline of F. V. Var. (3. PI. XIX. 194 /3. 



Genus 22. PLEUROSIGMA, W. Sm. 



Frustules free, elongated ; valves convex, sigmoid, with a central lon- 

 gitudinal line, and nodules at centre and extremities, striated; 

 striae resolvable into dots, which are frequently hexagonal. 



The sigmoid form of the valve more or less present in all our native 

 species, at once distinguishes this genus from its allies ; but even in 

 the absence of this feature, which I have noticed to be wanting in at 

 least one species, discovered by M. De Brebisson, on the coast of 

 Normandy, the structure of the siliceous valve retains its character- 

 istic peculiarities. The striae are invariably so closely arranged, that 

 their discovery and resolution are among the most difficult operations 

 in microscopy. Their true character has been, in consequence, fre- 

 quently mistaken ; some observers having considered these appear- 

 ances of striae to arise from series of perforations, and others from 

 rows of beads, or minute elevations. With the latter I have been 

 disposed to coincide, until, aided by the careful manipulation and 

 excellent object-glasses of Mr. Richard Beck, who has shown me 

 the hexagonal outline of these supposed beads, I have been led to 



