The Dicdoms of the London Clay. By F. Kitton. 385 



am quite sure that since you have found diatoms in the Lower 

 Eocene, and I in the carboniferous rocks, geologists will now expect 

 to find them preserved in other and older formations, and that they 

 will be obliged to confirm my observation that in the order of the 

 Diatomacese the species remain constant in their characters." 



As a result of many experiments, my opinion is that in a cer- 

 tain proportion of the diatoms the pseudomorphic process has not 

 been completed. On this point Dr. Bossey says, " I fully concur 

 with you in the belief that the silica in some cases remains." 



Bemarks on the above Ba]jer, tvith a List of the Genera. 

 By F. Kitton, Hon. F.K.M.S. 



In the month of December, 1879, I received a letter from Mr. 

 Shrubsole, asking me to give my opinion of some minute disks 

 (supposed by him to be diatoms), which he said I should find in 

 the material sent for examination, and which he informed me was 

 obtained by washing the London clay. I carefully examined with 

 a ^ objective many trial dips of the lightest parts of these cleanings, 

 but saw no trace of diatoms (excepting a recent frustule or two of 

 Melosira moniliformis, introduced by accident). As I was not at that 

 time aware that diatoms had been found in a mineralized condition, 

 I did not examine the heavy dark-looking debris, which subsided so 

 rapidly, and appeared so unlike any diatomaceous material with 

 which I was acquainted. I therefore wrote to Mr. Shrubsole, in- 

 forming him of my non-success. 



In his third communication he gave me the clue which enabled 

 me to detect the character of these discoidal bodies, and I was sur- 

 prised and delighted to find that they were undoubtedly diatoms, 

 but in a state so totally unlike any other fossil diatomaceous de- 

 posit, that any diatomist might be pardoned for not detecting them. 

 I at once wrote to him, congratulating him on his discovery. I 

 have since then received many samples of crude and cleaned 

 material, which has enabled me to identify the genera, and in some 

 instances the species, to which the diatoms belong. 



The first form I found was a Coscinodiscus. It was of a dark 

 bronze colour, and when examined as an opaque object with f ob- 

 jective and Lieberkiihn, the cellulation was distinctly visible. A 

 further search in the material (this was from the Sheerness well) 

 afforded many specimens, both perfect and fragmentary, of at least 

 two species of Coscinodisci, all of them being frustules. In samples 

 from other localities I found mixed with the frustules many 

 perfect valves, the outer surface of which were of the dark bronze 

 colour before mentioned, the inner surface having a brilliant 

 golden appearance, exhibiting all the lustre of a highly polished 

 gold plate, upon which, radiating from the dark central umbilicus, 



