BY G. I. PLAYFAIR. 125 



Var. FLUviATiLis, n.var. (PI. v., f.24). 



Forma striis brevibus e denticulationibus marginalibus cuneatis 

 orientibus. 



Diam. 15-18, areas centr. 8//. Striae in ambitu circ. 30. 



Lismore, all five forms (2, 20); var. convexii (2, 13, 17, 18, 20, 

 21, 22); var. quadrata (2, 5, 20, 21, 22); vaiV.fluviatilis (2, 5, 20). 



Genus Hydrosera Wallich. 

 Hydrosera triquetra Wallich. (PI. v., f.26). 



Long, (latere cingul.) 75-130; lat. 80-100; diam valv. 80 /x. 



Lismore (16, 19, 21). 



The valves are strengthened internally by septa across the 

 salient C? and intermediate) angles, the lower, free, edges can be 

 seen in girdle-view as flying arches. The two smooth vertical 

 bands are the extension of the papillae which appear at the 

 margin in optical section. These are really (as in all other 

 diatoms where they appear) loops of the inner incrassate mem- 

 brane, which run right round the valve. The thin outer mem- 

 brane remains flat, but often breaks away, showing the loop 

 plainly as a marginal indentation. "Grunow and H. L. Smith 

 unite the genus Hydrosera with Pleurodesmiunn and Teiysinoe, 

 which have the same structure."— Van Heurck, Diat., p. 453. 

 The latter relies on the triangular valve as a generic character. 

 I have not found any biradiate forms in the river. All three 

 genera have this in common, that they consist of marine forms 

 which are just as often found in the fresh water of rivers and 

 far above all tidal influence. 



Genus Coscinodiscus Ehr. 

 CosciNODiscus LACUSTRis Grunow. 

 Diam. 34-78; striae 8-9 in 10 /x; puncta circ. 10 in 10/x. 

 The puncta may be noted arranged in many different ways, 

 but most of these are merely transitory arrangements due to 

 growth. In the perfect form, the puncta are in regular radiat- 

 ing lines, but frustules are common in which tliey are disposed 

 quite irregularly or in fascicles with shorter rows filling the 



