Meridionacece 283 



two false longitudinal septa, well seen in the rectangular girdle- 

 view, and which are perforated by three openings, one central and 

 two polar. The valve- view is oblong-lanceolate, with a slight 

 median swelling ; its surface is transversely striated with indications 

 of a pseudo-raphe and slight traces of nodules. 



D. Balfouriana Grev. is a rare British Diatom the frustules of which 

 reach a length of 30/z. 



Genus Denticula Ktitz., 1844. The frustules are more or less 

 tabular, solitary or attached to form short filaments. There are a 

 row of imperfect transverse septa in each valve, which appear as 

 capitate marginal ribs in the subrectangular girdle-view. The 

 valve-view is lanceolate and the septa appear as strong transverse 

 ribs, between which are delicate punctate striae. The face of each 

 valve is carinate and for this reason Denticula has been associated 

 with Nitzschia. In the region of the girdle, and between it and 

 the valve on either side, is a longitudinal septum with a row of 

 perforations, the edges of these perforations being fused to the 

 transverse septa. There is no raphe or pseudo-raphe. 



D. elegans Kiitz. and D. tennis Kiitz. (fig. 130 C and D) are both frequent 

 amongst mosses on wet or dripping rocks. Valves up to 45 ^ in length. 



Family 2. MERIDIONACE^I. 



The frustules are more or less rod-shaped and cuneiform, and 

 are either united by their valve-faces to form a flat spiral filament, 

 which often makes two complete turns, or they are disposed at the 

 extremities of a branched system of gelatinous stalks. In both 

 the valve- and girdle-views the frustules are symmetrical in 

 relation to a longitudinal axis, but asymmetrical about a transverse 

 axis. In some there are two (sometimes more) false longitudinal 

 septa, but no transverse septa; sometimes these are confined to 

 the broad end of the frustule (Licmophora) and at other times 

 they extend the whole length of the valves (Climacosphenia). In 

 others there are no false longitudinal septa, but numerous short 

 transverse septa pass across the keeled face of each valve (Meridian). 

 There is no raphe, but there is a structureless sagittal line which 

 indicates the first commencement of a pseudo-raphe. Reaching 

 almost up to this clear line are numerous transverse striations of 

 a very delicate character. The chromatophores are scattered in 



