86 



BRITISH EOCENE ELORA. 



pervirens preserves the spiral, scale-like leaves for a longer or shorter distance, according 

 to climate, at the base of each branchlet, while S. gi(jantea sometimes assumes the disti- 

 chous arrangement. But besides this, the leaflets are not in two rows as they are in 

 Taxodiura, for their bases are decurrent and spirally arranged round the stem, becoming 

 a short distance from their attachment twisted to one side, flat, and expanded, and 

 crowded into two lateral rows, apparently in order that their surfaces may be exposed as 

 fully as possible to sun or moisture ; and with frequently an additional row lying centrally 

 along the upper surface. 



It has been surmised that the genus became well-nigh exterminated during the 

 Glacial Epoch, and has been most strangely preserved in two isolated spots, perhaps out- 

 side its original range, where the moderating influence of the Pacific had permitted it to 

 survive upon lofty spurs of mountains, while the valleys were filled with glaciers. Fixed 

 on exceptionally favorable stations and with congenial soil, the existing species may have 

 slowly adapted themselves to a temperature far more genial than that supported by their 

 ancestors.^ In adapting themselves since the Glacial Period to ever increasingly favor- 

 able conditions, they may possibly have acquired that stupendous habit of growth which 

 makes them the giants of vegetation. 



Sequoia is thus seen to be a genus which apparently first became differentiated during 

 the Cretaceous, and widely spread and adapted to a great range of climate during the 

 Tertiary, when it contained species which differed greatly from those that have survived. 

 The needle-leaved type is the most ancient, but this became dimorphic during the 

 Cretaceous in high northern latitudes, where it was finally superseded by the distichous 

 type. The latter appears even now to grow more imbricated in warmer temperatures. 

 The former ranged from hot to quite temperate climates during the Tertiaries, but not 

 into such northern countries as Greenland and Spitzbergen. 



Sequoia CouTTsiiE, Heer. PI. VI, except fig. 18. 



Seutjoia CouttsijS'. Lignite of Bovey-Tracey, Heer and Pengelly, Phil. Trans., 1863. 



Taxodium dubium. Fossile Flora von Bilin, Ettingshausen, p. 34, pi. x, 1866. 



NoN S. CoDTTSi^, Heer. Flora fossilis artica, and Miocane baltisclie Flora ; Baily, 



2nd Report on Antrim Flora, British Association 



Reports, 1880. 



Bovey-Tracey, Bournemouth, Hempstead, and Gurnet Bay. 



The leaves are alternate, spirally arranged, short and imbricated, keeled exteriorly, 

 concave on the inner surface, decurrent at base, with the apex free and frequently 



1 In estimating the minimum temperature required by the fossil Sequoise we may be misled through 

 relying too exclusively on the data furnished by existing species. 



