1897-98.] On the Diatomacece. 357 



find them covering some of the stones with what appears to 

 be a brownish slime, or you will find them adhering to grasses 

 on the margin of the stream, or to dead grasses floating in 

 pools of clear water ; you will find them adhering in gelatin- 

 ous masses to the faces of rocks or the sides of caves where 

 there is any water trickling down. In a word, wherever 

 there is slowly running or trickling water, you will find fresh- 

 water diatoms. The prevailing colour of these diatoms in the 

 mass is golden or dark-brown. 



Marine forms you will find growing on the slender alga? in 

 pools among the rocks on the sea-shore. A particular genus 

 (Licmophora) you will find growing abundantly on the upper 

 part of the slender, hair-like Litosiphon pusillus, imparting 

 to that pretty alga a somewhat cloudy, dirty appearance. 

 Again, where there is a bed of mud which is covered by the 

 tide at high water, such as some of the mud-beds on the 

 shore at Cramond, you will find a beautiful carpet of diatoms 

 glowing with a dazzling sheen as the rays of the sun fall 

 slantingly upon them ; while on boulders in the shallow water 

 on the sea-shore you will find the mucous tubes of the 

 Schizonema. Another excellent soil for diatoms is the mud 

 in pools filled by the tide at high water on marshy ground 

 along the sea-shore. Some of the best gatherings of Pleuro- 

 sigma I have ever made were got in such pools in the 

 marshy ground on the sea-shore near the North British Bail- 

 way station at Montrose. Again, where there is a burn 

 falling into the sea, with its outlet somewhat closed by 

 shingle, so that a pool affected by the tide is formed, and in 

 which some twigs may be floating, you will likely find on 

 these twigs a rich harvest of diatoms. 



As the algse to which the diatoms adhere, and probably the 

 diatoms themselves, form part of the food of certain fishes 

 and of some mollusca, some excellent specimens have been 

 found in the stomachs of these creatures. The task of search- 

 ing for them is, however, a laborious one. 



I have hitherto been speaking of living diatoms. There 

 are, however, in many parts of the world, large beds of them 

 in a fossil state. In Scotland there are several of such 

 deposits, both on the mainland and in some of the islands on 

 the west coast. The one of which I have personal knowledge, 



