CULTIVATION OF DIATOMS. 43 



Lacustrine Deposits. These deposits are produced in lakes, ponds, 

 and rivers. Strictly speaking, they are not fossils but rather recent 

 deposits, since the greater part of the diatom forms contained in them 

 are identical with living species. They, however, sometimes belong to 

 the tertiary and also quaternary periods. 



These deposits are pulverulent, and when dry they are remarkably 

 light. They are either quite white or grey, according to the quantity 

 of organic matter which they contain, and have been said to resemble 

 powdered starch. Their dampness when fresh usually deepens their 

 colour which becomes lighter as they dry. These should be gathered 

 in exactly the same way as marine deposits. As these beds are seldom 

 of any great extent (they often become soon obliterated or covered 

 up) it will be well to secure a good supply of the material whenever 

 the opportunity presents itself. If any shell, wood, or other organic 

 remains be found dispersed through the deposit, or overlying or 

 beneath it, they should also be collected, and their position recorded 

 on the label. 



Deposits of fossil diatoms are numerous, and are found in very 

 various localities. The most ancient is that found in 1878 by Mr. 

 Shrubsole, of Sheerness-on-Sea, in the London clay, which belongs to 

 the Lower Eocene (Tertiary period). Some of the diatoms found by 

 Mr. Shrubsole are encrusted with pyrites, and some have even been 

 transformed into pyrites. The number of species found is rather large, 

 the most common being Coscinodiscus perforatus (?) and Coscinodiscus 

 minor. (?) (') 



5. Cultivation of Diatoms. 



Every diatomist has at some time or another made diatom cultiva- 

 tions, that is to say, has tried to keep alive the species which he has 

 collected on his excursions, or which may have been accidentally 

 developed in the jars or aquariums of his laboratory. But the 

 attempt to keep any species alive for a protracted period rarely succeeds, 

 and then only under certain circumstances, which cannot be fully 

 ascertained. The longest cultivation, we believe, which has hitherto 

 succeeded is one which we have in our own laboratory, dating back 

 to 1886, since which time some Navicula didyma and Amphora duplex 

 have continued to multiply. 



( J )The Diatoms of the London clay, by W. H. Shrubsole, F.G.S., with a list of 

 species and remarks by F. Kitton, Hon. F.R.M.S., J.R.M.S. (1881), p. 381. 



