46 BRITISH DIATOMACEiE. 



2. Tabellaria fenestrata, Kutz. Frustules definite ; septa 2 from 

 each margin, equal. Length of frustule '0008" to -0046"; in- 

 flations of the valve nearly equal in width; striae as in the 

 last. v.v. 



Kutz. Bacill. xvii. 22. Raben. Siissw. Diat. x. 1. Biatoma fenestratum, 

 Lyng. Tent. lxi. E? Ralfs, Ann. vol. 11. pi. ix. 4. ad specim. authen. 

 in herb. Jenn. 



Fresh water : often with the last. Wareham, Dec. 1849 ; Studland, Dor- 

 setshire, Sept. 1853 ; Maam, Galway, and Botanic Garden, Belfast, July 1853 ; 

 Grassmere, Westmoreland, and Rackham, Sussex, Aug. 1853 ; River Lee, near 

 Cork, April 1855, W. Sm. Rackham, Oct. 1842, Mr. Jenner. Ormesby, 

 Norfolk, Aug. 1853, Mr. Brightwell. Haverfordwest, March 1854, Mr. 

 Okeden. Ulverstone, Lancashire, Aug. 1854, Miss E. Hodgson. Lough 

 Mourne and Lough Island Reavey Deposits. Dolgelly Earth. (Falaise, 

 M. De Brebisson. Pic de Sancy, Mont Dore, elev. 6000 feet, June 1854, 

 W. Sm.) 



Very like the last in the S. V. of its valve, especially in small specimens, 

 but always to be known from it by the definite structure of the frustule. 



Plate XLIII. 317. 



Genus 45. AMPHITETRAS, Ehr. 



Frustules cubical, cohering into a zigzag attached filament ; con- 

 necting membrane cellulate, imperfectly annulate, indefinite ; 

 valves quadrangular, cellulate ; cellules circular, radiate, or irregu- 

 larly concentric, inconspicuous at the angles of the valve. 



The cubical outline of the frustules distinguishes the species 

 included in the present genus from all our other native forms. The 

 irregular development of the connecting membrane and its annulate 

 structure ally it to the several genera lately described ; but the entire 

 absence of internal septa is a character which places Amphitetras 

 apart from Tabellaria, and, together with the zigzag of its filaments, 

 forbid its union with Rhabdonema or Tetracyclus. The projection of 

 the connecting membrane beyond the suture of the valve is a cir- 

 cumstance which meets us for the first time in Amphitetras ; it will 

 also be found in Biddulphia ; and the enlarged appearance which is 

 thus given to the central portion of the mature frustule is a con- 

 spicuous character in these genera. 



