28G KEY. E. o'meara on peculiar forms of navicula. 



Navicula spiralis (fig. 3). — Valve constricted, abruptly 

 rounded off towards the broadly lanceolate apices ; length 

 •0050; greatest breadth "0015; breadth at constriction 

 •0014. Central nodule quadrangular, longer than broad, 

 with three linear projections on each side. Median line 

 slightly sigmoid, close to which is a narrow band of costae, 

 apparently arranged in a spiral form. Costa; marginal, 

 slightly radiate towards the apices, and nearly parallel in the 

 middle, leaving a free space between them and the spiral 

 band of costae before mentioned. The space between the 

 marginal costae occupied by a double row of minute parallel 

 punctate dots. 



Navicula unipunctata (fig. 4). — Valve constricted, abruptly 

 rounded off towards the broadly lanceolate apices: length 

 •0043; greatest breadth -0010; breadth at constriction ^0009. 

 Central nodule square ; median line slightly sigmoid. Costoe 

 distant, narrow, nearly parallel at the margin, and waved 

 towards the median line, near which they terminate, forming 

 a crenulated border. The space between the costae occuj^ied 

 by a single row of minute punctate dots, which extend only 

 a short distance from the margin, and form a narrow border. 



Navicula bipunctata (fig. 5). — Valve very slightly con- 

 stricted, abruptly rounded off towards the broadly lanceolate 

 apices; length '0044; greatest breadth •OOl^; breadth at 

 constriction •OOllS. Central nodule is square, hollowed 

 inwards on all sides ; median line slightly sigmoid, having a 

 narrow unstriated space on either side. Costae radiate 

 towards the apices for half the distance, and for the rest 

 radiate towards the central nodule. The spaces between the 

 costae for about half their length occupied by a double roiv of 

 minute punctate dots. 



This form bears a strong resemblance to N. unipunctata, 

 but is less constricted, longer and broader, and the border 

 formed by the interstitial dots is broader, the dots being in 

 double series, while in N. unipunctata the dots are in single 

 series. 



Navicula plutonia (fig. 6). — Valve much constricted; length 

 •0031 ; greatest breadth -0011 ; breadth at constriction •OOOO. 

 Central nodule square ; median line arched from centre to 

 the apices. Costae tolerably broad, radiate partly towards 

 the apices, partly tOAvards the central nodule, nearly reaching 

 the median line. The spaces between the costae for the 

 greater part of their length occupied by a single row of small 

 moniliform dots. 



The practical result arising out of the discovery of the 

 forms here described, as it strikes my mind, is to force us 



