BRITISH DIATOMACE^E. 3? 



The figure given by Kutzing, Bacill. pi. 18. fig. v. 3, representing a 

 smaller frustule attached to a larger, is manifestly an error arising from 

 the accidental juxtaposition of two frustules of diiferent filaments. 



The stria; on the surface of the valve are exceedingly delicate, and 

 require a higher power than the one we have usually employed to 

 bring them distinctly into view. 



Striatella unipunctata, Ag. Annuli 12 to 36, 12 in *00l"; septa 

 alternate. Width of filament -0022" to -0032". Breadth of valve 

 •0006" to -0009" ; stria; 68 in -001". v.v. 



Ag. Consp. p. 61. Kiitz. Bacill. xviii. 5. Ralfs, Ann. vol. 13. pi. xiv. 6. 

 Harv. Man. p. 109. Fragilaria unipunctata, Lyng. Tent. Ixii. G. Grev. 

 B. F. p. 405. Achnanthes unipunctata, Grev. S. C. F. tab. 287. Tes- 

 sella Catena, Ehr. Inf. xx. 7. Prit. Anim. iii. 180. Tessella pedicellata, 

 Dujar. Inf. xx. 14. 



Marine. Torbay, July 1846 ; Lulwortb Cove, Dorsetshire, Aug. 1847 ; 

 Jersey, Aug. 1852 ; Newhaven, Sussex, Sept. 1852 ; Larne and Belfast Bays, 

 July 1853; Poole and Weymouth Bays, Sept. 1854; Cork Harbour, July 

 and Oct. 1855, W. Sm. (Mediterranean Sea near Marseilles, May 1854, 

 W. Sm.) 



Plate XXXIX. 307 : margin of a frustule, 307 x. 



Genus 41. TETRACYCLIC, Ralfs. 



Filament compressed, with a central inflation, free ; frustules annu- 

 late, indefinite ; annuli plane ; septa alternate, equal ; valves 

 inflated at the centre, cruciform, costate. 



This genus, established by Mr. Ralfs in 1843, is easily distinguished 

 from most of its allies. Rhabdonema minutum and Tabellaria floccu- 

 losa are the only forms with which the species it includes might be 

 confounded. It resembles R. minutum in the F. V. of its filament, 

 but differs from that species in its cruciform and costate valve and 

 equal septa, and is easily distinguished from Tabellaria flocculosa by 

 its continuous filament. 



The costae in the present genus are usually pervious, and curved ; 

 occasionally, however, they are direct or interrupted. 



The genus Biblarium, constituted by Ehrenberg in 1845, appears 

 to differ from the present merely in the solitary character of its 

 frustules, and this character arises from the fossil nature of the 



