INTRODUCTION 21 
ridge Clay, and some doubtful Dasycladacew from 
other beds, there is no valid evidence of other forms 
until the chalk is reached, with its Diatomacee and 
Lithothamnion in the Senonian (Cretaceous) beds, 
Coccoliths, and Rhabdoliths, There come next the 
extensive Tertiary deposits of diatoms, and the 
beautiful verticillate Stphonee of the same age 
described by Munier Chalmas; the coralline Litho- 
thamnion and the fresh-water Characee. The later 
beds furnish little of interest except Quaternary 
diatomaceous remains. There is room for dis- 
appointment in the failure to find indubitable 
‘records in earlier rocks of plants of such primitive 
type as the Algz, and it is startling to find such 
forms as the diatoms suddenly burst upon geological 
history in a profusion of genera and species, many 
of which survive in their specific forms from 
their first appearance to this day. From all that is 
known of. the Silurian rocks, for example, the 
discovery of diatoms in them would appear to be 
highly probable, but research has failed to discover 
them. However intractable and therefore suited 
to preservation their siliceous shells may be, the 
existence of conditions under which this substance 
would become fugitive is probable, and the gap, 
though significant enough, is not more so than the 
absence of JMMuscinew, for example, from the coal 
measures. It but emphasizes the imperfection of 
the geological record of plant history, and points to 
caution in generalising from an insufficient array of 
facts, more than it indicates argument in favour of 
any particular sequence of primitive plant forms. 
