lxxxvi PKOCEEDINGS OP THE GEOLOGICAL SOCIETY, [vol. lxxix, 



considered it to be a Thallophyte with algal affinities ; but the 

 fact that the spores were provided with a protective cuticle, and 

 were therefore adapted to dispersal by wind, is in favour of 

 Dr. Hickling's suggestion that ParJca may be the spore-bearing 

 phase of a plant which grew on lacustrine rnud exposed to the air. 

 It may be compared with some existing liverworts, such as Riccia. 

 Whether in this Lower Devonian genus we have a complete 

 organism or the spore-bearing portion of some unknown plant, it 

 is of great interest as one of the oldest-known plants that has 

 revealed its minute structure. It existed during the latter part 

 of the Silurian Period, and flourished in the early part of the 

 Devonian : its first appearance coincided, so far as we know, with 

 the earliest stage of the invasion of the land by aquatic plants. 

 The spores of ParJca may well have been among the first of 

 countless millions of wind-borne plant-dust which has strewn the 

 earth's surface since the arrival of terrestrial vegetation. Al- 

 though in the grosser features spores of very different races of 

 plants are amazingly alike, their living protoplasm was endowed, 

 in the course of geological history, with ever-changing and widely 

 divergent potentialities. 



Devonian Floras. 



The Devonian Period demands especial attention, as it intro- 

 duces us to something more than mere doubtful fragments of 

 terrestrial plants. It is not only the external features of some 

 Devonian fossils that lead us to identify them as dwellers on land; 

 but, in one favoured locality, the wonderful perfection of preserva- 

 tion makes it possible to correlate anatomical characters with the 

 actual conditions governing the life of the plants. 



In some regions of the world the piling-up of marine sediments 

 which characterized the earlier Palaeozoic Eras was continued in 

 orderly sequence into the Devonian Period. In the western, 

 southern, and central parts of what is now the continent of Europe 

 limestones and other strata rich in marine fossils indicate the 

 presence of a Devonian ocean. It is the records of another phase 

 of Devonian history that claim our attention. Before the close of 

 the Silurian Period movements of the Earth's crust were in- 

 augurated which gradually increased in intensity, and in the earlier 

 part of the Devonian Period culminated in the replacement of 

 wide stretches of the Silurian sea by mountain -ranges. The 

 Caledonian chain connected by a series of lofty summits the 



