447 



tions of existing to extinct molluscs, and that the Terebratula of the 

 Sihirian epoch is found to exist at the present day. 



But in the case of Diatoms, there is reason to think that the 

 whole of the species which occur fossil will, ere long, be detected in 

 the recent state, just as has occurred in the case before us, N. prce- 

 texta, which, it must be remembered, occurs in the oldest Diatoma- 

 ceous deposit yet described, and along with forms, nearly all of 

 which I have actually found in the Clyde. 



It is at all events certain, that a very largo proportion of the 

 Diatoms found in the fossil state also occur in the living state, and 

 that every day adds to their number. 



There is at present no good evidence of the existence of Diatoms 

 earlier than the Chalk, if so early. But we must not forget that 

 the shells of Diatoms appear to be altered by long contact with car- 

 bonate of lime, so that they may have existed at one time in the Chalk. 

 We find them, however, in spite of the action of calcareous matter, 

 in the Chalk Marls of Meudon and of Caltanisetta, which are rather 

 more recent than the Chalk, and probably about the age of the Clay 

 Marl of .^gina. If, as I believe, no Diatoms have become extinct, 

 this may, perhaps, depend on their minute size and extreme simpli- 

 city of structure, which probably render them more indifferent to 

 climatic changes than more highly organized and larger beings. 



We have evidence, to a certain extent, that this is the case ; for 

 by Ehrenberg's figures it appears, that in gatherings of recent Dia- 

 toms, from all parts of the world, in every possible variety of cli- 

 mate, the majority of the species are identical with our own. 



Diatoms, therefore, are not materially affected by existin<T dif- 

 ferences of climate, and have probably been as little affected by the 

 geological changes which have occurred, at all events since the pe- 

 riod of the eocene deposits. 



To return to the new forms. 



Group II. 



Cocconevies. 

 The number of new forms in this group is not large, but they are 

 all interesting. They are as follows : — 



23. Cocconeis distans, W.G. 



24. ,. dirupla, n. sp. 

 ■i^'^- I. ornata, n. sp. 

 ^- .> nitida, n. «p. 



27. Ck>eeoneis pseudomarginata, n. sp. 



28 



major, n. sp. 



29. ,. splendida, n. sp. 



2 2 



