616 J. G. GRENFELL. 



small isolated Melosiras and Cyclotella Kiitzingiana have 

 these pseudopodia normally. At Stanstead, Heytesbury, Kew, 

 and Eastbourne there was no trace of Archerina. The reason 

 for mentioning this is that it was suggested that the external 

 protoplasm of Archerina migrated on to the diatoms. 



The pseudopodia of the first gathering and of the Stan- 

 stead ones were easily seen for a part of their length with 

 a ^ object-glass, magnifying nearly 400 diameters; but in the 

 case of the Heytesbury Melosiras and most of the Cyclotellas 

 from the Botanical Gardens and Kew they are generally in- 

 visible, even when specially looked for. I think this helps to 

 explain why they have not been found before. 



To study the whole length of the pseudopodia I found it a 

 good plan to dry the material on a cover-glass, which could 

 then either be mounted dry or stained or roasted. All the 

 figures on PI. XLI are from specimens thus treated. Quite 

 lately I have stained and mounted some of the Kew gathering 

 without drying. The results I hope to give in a future paper. 

 So far they have simply confirmed the main conclusions at 

 which I had already arrived. 



The principal points to notice in the structure of the pseu- 

 dopodia are these : they are fairly stiff, and are non-retractile 

 to ordinary observation. 



The length varies in Cyclotella from two and a half to six 

 times the width of the valves. In the Heytesbury Melosiras 

 this reaches fully nine times the width. They are very per- 

 manent; a slide prepared in April by simply sealing the 

 diatoms in the water in which they were found showed the 

 pseudopodia apparently unchanged after five months. The 

 dried slides show that the great majority of the pseudopodia 

 are arranged fairly symmetrically round the margin of the 

 valves. This is best seen in side view (figs. 6 and 7). 



In slides of the Kew gathering prepared in the wet way a 

 great many Cyclotellas show a series of small tooth-like pro- 

 jecti(i<ns of protoplasm round the margins of the two valves, 

 arranged as regularly as the teeth of a circular saw. These 

 projections are the thicker bases of the delicate pseudopodia. On 



