BRITISH DIATOMACE^E. 59 



7. Melosira Westii, n. sp. Frustules subglobular ; valves conical, 

 with truncated apices, and a sutural and median siliceous ring. 

 Breadth of filament -0008" to -0016". v.s. 



Marine. Stomach of Pecten, Coast of Sussex, W. Sm. ; detected by Mr. 

 West, March 1852. 



Plate LII. 333. 



Genus 50. ORTHOSIRA, Thwaites. 



Filament cylindrical, of numerous frustules, continuous, attached or 

 free ; frustules and valves cylindrical ; junction-surfaces plane, line 

 of junction usually spinous or denticulate. 



1. Orthosira arenaria, W. Sm. Filament curved ; cell- cavity sub- 

 spherical; valves with a subsutural line of puncta; line of junction 

 with numerous minute denticulations or spines ; junction-surfaces 

 marked with radiating striae. Breadth of filament '0026" to 

 •0051". v.v. 



Melosira arenaria, Moore in Ralfs, Ann. vol. 12. pi. ix. 4. Kiitz. Bacill. 

 xxi. 27. Raben. Siissw. Diat. ii. 5. Melosira arenosa, Hass. Alg. xciii. 

 2 & 3. Gallionella varians, Ehr. Inf. xxi. 2. G. varians, undulata, et 

 biseriata, Ehr. Microgeologie. 



Freshwater. Wareham, Dorset, June 1848, W. Sm. Westerham, Kent, 

 Dec. 1843, Mr. Jenner. Near Belfast, Dr. Dickie. Surrey, Mr. Capron. 

 Berwickshire, Dr. Arnott. Queen's Park, Edinburgh, April 1854, Dr. Gre- 

 ville. Lough Mourne and Peterhead Deposits, &c. (Falaise, M. De Br6- 

 bisson. Source of the Loiret, near Orleans, June 1854, W. Sm.) 



In this and the two succeeding species, the cell-cavity of each frustule is 

 more or less spherical, while the siliceous epiderm is cylindrical; hence the 

 margins of the filaments are direct and continuous, and no depressions are 

 conspicuous at the points where the frustules are united. 



The filaments of O. arenaria, when dry, are of a purple colour, and exhibit 

 a rich metallic lustre. They run no risk of being confounded with those of 

 any other form ; and it is difficult to account for the confusion into which 

 Ehrenberg has fallen with regard to the present species, to which he seems to 

 apply indiscriminately the names of Gallionella varians, G. biseriata and 

 G. undulata. 



Plate LII. 334. 



2. Orthosira marina, W. Sm. Filaments direct; cell-cavity sub- 



