218 PROCEEDINGS OF THE 



fore, neither a species of Nitzschia nor of Tryhlionella, but has 

 been correctly removed by Dr Lewis to Surirella, and called by 

 him ^S*. intermedia, and chiefly differing from his S. anceps by 

 the latter having conspicuous strise. Smith's section of a frustule 

 in the centre of Plate X. is intended to illustrate TrijUionella, but it 

 is a section of Surirella. 



Several species of Nitzschia have the zones marked by num- 

 erous lines. These seem to arise from the inability of the 

 frustule to self-divide at the proper time (this may be from want 

 of silex in the water, or otlier causes), and these abortive attempts 

 may go on, the zone receiving its increments and widening, until 

 at last the new valves are formed. These lines, and the breadth 

 of a zone, especially when marked by such lines, are therefore 

 accidental, and cannot be admitted as marks to distinguish species. 

 N. lanceolata of Smith is a beautiful illustration of this kind of zone. 

 When boiled in acid, it separates at these lines; but when ex- 

 amined before the valves be disjoined, tlie true structure may bo 

 easily seen. By some oversight. West, in making the figaires of 

 that species for Smith's synoi:)sis, has represented the lines as if 

 on the valves themselves (in place of being merely seen through 

 the valves), and the same error takes place in the specific char- 

 acter. The lines mentioned there are not on the valve, but on 

 the zone, where they vary from 8 to 35 in '001. N. incrustans of 

 Grunow seems to be a small, somewhat distorted form of this 

 species, less silicious tlian even the typical fonn is, and more 

 common. These complex zones, as Gregory called them, are not 

 uncommon in other genera, as in Amphiprora and Amphora, where 

 Gregory considered them so important that he divided that genus 

 into sections from their presence or absence — an unfortunate 

 mistake, which will prevent many of his species being recognised, 

 unless chance presents other specimens in a similar state. The 

 same sort of zone occurs in Sckizonema GreviUei (and perhaps in 

 other species), especially when, from some cause, the usual en- 

 veloping tube is not formed. Navicula libellus of Gregory is 

 Schizonema GreviUei in this state. His N. rhomhica is a smaller 

 form of the same. The allied genus Ainphi])leura is in an ano- 

 malous condition; its name alludes to supposed longitudinal ridges 

 on the valves. This is entirely a deception; there are no ridges 

 or keels on the valves, except in A. sigmoidea, and that is a 

 genuine species of Nitzschia, and is a new variety of N. sigma, 



