of British species oj the yenus Pieurosigina. 5 



from unity when each step doubles upon the preceding. Thus, 

 supposing self-division to be perfected in twenty-four hours, 

 a single frustule will in one month have increased to up- 

 wards of a thousand millions ! But it is also certain that the 

 Naviculece, like some other DiatomacecB, have a specific mode of 

 reproduction, since we often find frustules in various stages of 

 growth, as is evident from the diversities of their size (PL I. 

 figs. 7 & 9; PI. II. figs. 1, 2 & 3, 9 & 10, 11 & 12), and fi-om 

 the greater dehcacy of the striae in individuals of the same species ; 

 circumstances which are incompatible with the process of self- 

 division, where the half- new frustules must of necessity be pre- 

 cisely counterparts of the old. The mode in which the germi- 

 native power is renewed, when exhausted by self-division, remains 

 still to be discovered ; it will probably be found to be a process 

 analogous to that of conjugation in the Desmidieaj and which 

 is known to obtain in some of the families belonging to the 

 DiatomacecB. In the genus under review, the most careful search, 

 at all seasons, and during every stage of growth, has failed in 

 any case to recognise the slightest indications of such a phseno- 

 menon. 



It may be as well that I should repeat, that the descriptions 

 I am about to give depend upon observations made with a 

 ^-inch object-glass and eye-piece giving together a power of 

 400 diameters, and that the figures, unless otherwise stated, are 

 drawn with the camera lucida exactly to this scale. The colours 

 mentioned are those of the desiccated valves not immersed in 

 balsam, or any other refracting medium, and are of course only 

 loose and imperfect modes of designating shades and varieties of 

 aspect, which it is impossible to define with accuracy. 



Pleurosigma. 



Section I. Beads alternate, strice oblique : all marine. 



. 1. Pleurosigma formosum, n. sp. Valve linear-lanceolate, gra- 

 dually attenuated to the somewhat obtuse ends, twisted; 

 median line broad, not central : colour bright chestnut-brown. 

 Average length of valve ^^^ of an inch, greatest breadth of 

 ditto ^J-y of an inch. Oblique strise jo^o (J ^^ ^^ ^^^^ apart 

 [v. v.). 



Shoreham Harbour, 1850. 



This species is well distinguished by the position of its me- 

 dian line, which, owing to a twist in the valves, appears nearly 

 to coincide with the edge for a considerable distance at either 

 end, and then crosses the valve in a diagonal direction, giving a 

 peculiarly beautiful appearance to the frustule, especially when 

 mounted in balsam. Mr. Kingsley has furnished me with a 



