484 PROFESSOR GREGORY ON 



soils, I have always found a very great majority of common British species. As 

 an example, I may specify two soils particularly rich in Diatoms ; one from the 

 Sandwich Islands, the other from Lebanon. The former was quite like an ordi- 

 nary fresh- water gathering, the latter resembled a poorer material. In both, I 

 have great doubts whether all the numerous species are not identical with our 

 own. Some few of the species, indeed, are not to be found in Smith's Synopsis ; 

 but most of these have been described by others, or by myself, as British forms, 

 since that work appeared. 



So far, therefore, as the greatest actual differences of climate are concerned, 

 Diatoms are apparently not affected ; as in the cases just mentioned, it is impos- 

 sible to distinguish the exotic specimens from British ones. 



If, therefore, Diatoms did not exist earlier than the Eocene period, it is quite 

 conceivable that none of them may have become extinct. 



I have already stated that the Clay Marl of yEgina is supposed by some to be- 

 long to the formation next below the Eocene, that is, to the latest Cretaceous beds ; 

 but that there is no satisfactory evidence of Diatoms in any earlier formation. If we 

 admit the Xanthidia to be Diatoms, these forms are known to occur in chalk 

 flints. But the Xanthidia are not usually regarded as Diatoms, and I have not 

 seen, either in flint or in chalk, any admitted or recognised Diatoms. 



Eheenberg figures many microscopic forms from the Chalk and older strata, 

 some even from the Silurian Greensand. But these older forms, at least so far 

 as are shown in the Microgeology, are not Diatoms, but either the siliceous Poly- 

 cystinese, or the calcareous Polythalamia, or, finally, sponge spicules. 



I admit that Diatoms may have existed in the Chalk or earlier, and that, by 

 a slow chemical change, they may have been destroyed, so that their form is lost, 

 the siliceous material alone remaining, whether alone or in combination. We may 

 even suppose that flint has been formed in part from the shells of Diatoms which 

 lived along with the Foraminifera or Polythalamia of that period. But these 

 are mere conjectures, and till Diatoms are found in the older strata, it must re- 

 main doubtful whether they existed previous to the Eocene period. 



The Chalk or Marl of Meudon, near Paris, and that of Caltanisetta, in Sicily, 

 exhibit a mixture of microscopic forms, calcareous and siliceous, including Dia- 

 toms. Here Diatomaceous shells, in contact with excess of calcareous matter, 

 have remained unaltered ; and if the Chalk of the true Cretaceous period had ori- 

 ginally contained Diatoms, it seems probable that they would have been found as 

 little altered as those of the newer beds just alluded to. 



On the whole, then, it is probable that the continued existence of all, or nearly 

 all, the known fossil species of Diatoms is the result, first, of their comparatively 

 late introduction, and secondly, of their small susceptibility to climatic changes, 

 arising from their minute size and very simple structure. 



12. Navicula Bombus, Ehr. PI. IX., fig. 12. Form much constricted in the 



