284 



BadllariecB 



the form of numerous small plates. Two auxospores arise from 

 two mother-cells. 



Genus Meridion Ag., 1824. The frustules are very similar to 

 those of the genus Diatoma. The valve-view is clavate, sometimes 

 with a subcapitate apex, and the girdle-view is cuneate with a 



Fig. 130. A and B, Meridion circular e (Grev.) Ag., from Shipley Glen, W. Yorks. 

 (x500). C and D, Denticula tennis Kiitz., from Wicken Fen, Cambridge 

 ( x 600). 



truncate base and apex. There are no false longitudinal septa, 

 but a number of imperfect transverse septa, which appear as very 

 short marginal costse in the girdle-view and as transverse costce in 

 the valve-view. Between these costa? are fine punctate striae 

 interrupted in the middle by a smooth line or pseudo-raphe. The 

 frustules remain attached after division, forming beautiful, fiat, 

 spiral filaments, which are free-floating. 



M. circulare Ag. (fig. 130 A and B) is often abundant in stagnant ditches 

 and ponds, and may frequently be obtained in pure masses. The var. con- 

 strii-tum (Ralfs) V. H. is also frequent. It is a Diatom which appears to be 

 most abundant in the early spring, often forming brown flocculeut masses 

 around submerged grass-leaves, etc. It is one of the prettiest of British 

 freshwater Diatoms ; length of valves up to 25 p. 



Family 3. DIATOMACE^. 



The frustules are elongate, rod-shaped or lanceolate in the 

 valve-view, and are united to form ribbon-like or zig-zag filaments. 

 There is no keel, but the valves possess strongly marked transverse 

 ribs which project inwards as more or less deep transverse septa. 

 There are no false longitudinal septa. The pseudo-raphe is con- 

 spicuous or it may be wanting. The girdle-view is rectangular. 



Genus Diatoma D.C., 1805. The valve- view is lanceolate or 

 linear, sometimes with capitate extremities, and is furnished with 



