32 BRITISH DIATOM ACEiE. 



median line, and central and terminal nodules ; striae obscure. 

 Length of frustule -0005". v.v. 



W. Sm. Ann. ser. 2. vol. xv. pi. i. 9. July 1854. 



Fresh water. Lasswade near Edinburgh, Dr. Greville. (Vaucluse, May 

 1854, TV. Sm.) 



Supp. Plate LXI. 381. 



Genus 39. RHABDONEMA, Kiitz. 



Filaments compressed, continuous, attached, or -stipitate; frustules 

 annulate, indefinite ; annuli plane, cellulate on their circumference, 

 forming internal septa ; valves elliptical, striate, with a median line, 

 striae moniliform. 



The present genus first presents us with a mode of increase in the 

 dimensions of the frustule, which we shall afterwards find occurring 

 in other filamentous forms, and which has caused no little perplexity 

 to the student of these organisms. As this phaenomenon has hitherto 

 frequently escaped the notice of writers on the Diatomaceae, and the 

 appearances it produces been misunderstood, it will be necessary to 

 enter into a somewhat lengthened explanation. 



In the species in which this mode of increase occurs, the valves are 

 similar in character to those of the other Diatomaceae with which we 

 are familiar, and are formed during self-division in the manner already 

 described (Introduction, vol. i. Section V.) ; but instead of the usual 

 repetition of the process of valve-formation, there detailed, we are 

 here presented with a subsequent intervalvular development, which, 

 not confined to the exterior of the frustule, projects a plate of silex 

 into its interior, forming a septum or partition extending towards, 

 but not reaching, the centre of the cell, and appearing as a com- 

 pressed rim, or annulus of silex, whose outer or larger circumference 

 follows the exterior outline of the frustule, and whose inner edge 

 bounds the free space, which serves as a channel of communication 

 between the chambers into which the cell is thus divided. This 

 process is either simultaneous, and the frustule definite, or success- 

 ive, and the frustule indefinite. In the first case, the annuli of silex 

 are formed during the production of the valves in the progress of 



