BRITISH DIATOMACE^E. 33 



self-division, and on every repetition of such production ; while in 

 the second case, the formation of the annuli is continued after the 

 production of the valves, and is repeated an uncertain number of 

 times before the recurrence of a new valve-production. The interior 

 of the frustules in both cases is divided by the annuli into chambers, 

 communicating with each other by the free spaces left between the 

 inner margins of the annuli, or by perforations in the septa ; con- 

 sequently the frustules, although apparently complex in structure, 

 still remain unicellular as in the more ordinary forms. This may 

 easily be seen by the distribution of the endochrome in the living, or 

 its aggregation into a simple mass in the dead frustule. It is to be 

 noted, however, that in the case of those species in which this increase 

 is indefinite, we do not find the regularity of operation which charac- 

 terizes self-division in other species ; the formation of the new valves 

 taking place with apparent indifference at any part of the elongated 

 frustule, that is to say, between any two of the annuli. This gives a 

 want of symmetry to the appearance of these forms, and enables 

 the observer to detect their true character. On a front view, the 

 septa, which project into the cell, are seen as costae, extending from 

 the margin towards the centre of the frustule ; and on a side view, 

 the spaces intervening between the convex interior edges of the septa, 

 or the perforations in the septa when such exist, appear as circular 

 or elliptical outlines traced upon the inner surface of the valve. These 

 appearances have received the name of " vittag " from Kutzing, and 

 " canals " from Meneghini, but either term appears to be unsuitable if 

 their true nature is considered. Ralfs, who was cognizant of the true 

 character of the septa, but who does not appear to have apprehended 

 the mode of their formation, accurately describes the appearances to 

 which they give rise (Transactions of the Botanical Society of Edin- 

 burgh, vol. ii. p. 22), as " produced by siliceous plates arising inter- 

 nally from the margin of the filament, and extending towards, but 

 not reaching the centre. The interior is thus divided into chambers 

 opening into a central space. When viewed laterally, the central space 

 has the appearance of a canal, especially as the inner edge of each plate 

 has a concave outline. This appearance is more striking in Tabel- 

 laria, Striatella tcenicpformis (= Grammatophora marina, Kiitz.), and 

 Tetracyclus lacustris, where all the plates are nearly equal ; but in 

 Striatella arcuata ( = Rhubdonema arcuatum, Kiitz.) and Tessella Catena 



VOL. II. D 



