PRELIMINARY NOTICES. 5 



plate affords a ready means of ascertaining the size of the specimen from 

 which each figure has been drawn ; but where much diversity has been found 

 to exist in the dimensions of the individuals examined, the extremes observed 

 have been noted in the text. 



The figures, unless when otherwise indicated on the Plates themselves, are 

 to be regarded as representations of the objects as seen under an amplifying 

 power of 400 diameters, and the details given may be taken as tests of the 

 defining capability of an instrument provided with a magnifying power of 

 that amount. In a few cases a lens of somewhat higher power has been 

 employed, to resolve and estimate the number of the strise, this designation 

 being applied in every case, not otherwise studied, to the transverse lines or 

 markings of the siliceous valves. 



The following British Diatomaceous deposits, recent or subfossil, are those 

 most frequently referred to : — 



Peterhead Deposit. Described in the Annals of Natural History, Aug. 1 848, 

 by Professor Dickie. 



Premnay Peat. Described in the Annals of Natural History, Aug. 1848, 

 by Professor Dickie. 



DoLGELLY Earth. Supplied by Chr. Johnson, Esq., Lancaster, 1850. 

 Noticed by him in the Annals of Natural History, June 1847. 



Marl, Co. Down. Supplied by J. M'^Adam, Esq., Belfast, 1849. 



Lough Island-Reavey Deposit. Supplied by the late William Thompson, 

 Esq., Belfast. Described in Magazine of Natural History for July 1839, 

 by Dr. Drummond ; and noticed in the Annals of Natural History, Feb. 

 1850, by the present writer. 



Lough Mourne Deposit. Described in Annals of Natural History, Feb. 

 1850, by the present writer. 



Cantyre Peat. Supplied by Prof. Balfour, and noticed in the Annals of 

 Natural History, April 1851 and June 1851. 



Raasay Earth. Fossil Earth from the Island of Raasay, supplied by Prof. 

 Dickie. 



A few species are referred to Foreign localities ; these references have been 

 determined by specimens distributed by the London Opticians or known to 

 microscopical observers, and for the genuineness of which the present writer 

 is unable to assign his personal authority ; — such notices may serve in some 

 degree to indicate the wide range of many of our native forms. 



