104 DIFFICULTIES AND SOURCES OF ERROR. [CH. 



corresponding appearance cannot be referred with certainty to 

 one special genus ; such casts are of no real scientific valued 



The old generic terms Artisia and Sternhergia illustrate 

 another source of error which can be avoided only by means 

 of a knowledge of internal structure. The former name was 

 proposed by Sternberg and the latter by Artis for precisely 

 similar Carboniferous fossils, having the form of cylindrical 

 bodies marked by numerous transverse annular ridges and 

 grooves. These fossils are now known to be casts of the 

 large discoid pith of the genus Gordaites, an extinct type of 

 Palaeozoic Gymnosperms. Galamites and Tylodendron afford 

 other instances of plants in which the supposed surface 

 characters have been shown to be those of the pith-cast. The 

 former genus is described at length in a later chapter, but the 

 latter may be briefly referred to. A cast, apparently of a stem, 

 from the Permian rocks of Russia was figured in 1870 under the 

 name Tylodendron ; the surface being characterised by spirally 

 arranged lozenge-shaped projections, described as leaf-scars. 

 Specimens were eventually discovered in which the supposed 

 stem was shown to be a cast of the large pith of a plant 

 possessing secondary wood very like that of the recent genus 

 Araucaria. The projecting portions, instead of being leaf- 

 cushions, were found to be the casts of depressions in the inner 

 face of the wood where strands of vascular tissue bent outwards 

 on their way to the leaves. If a cast is made of the 

 comparatively large pith of Araucaria imbricata the features 

 of Tylodendron are fairly closely reproduced^ 



A dried Bracken frond lying on the ground in the Autumn 

 presents a very different appearance as regards the form of the 

 ultimate segments of the frond to that of a freshly cut leaf. In 

 the former the edges of the pinnules are strongly recurved, and 

 their shape is considerably altered. Immersed in water for some 

 time fern fronds or other leaves undergo maceration, and the 

 more delicate lamina of the leaf rots away much more rapidly 

 than the scaffolding of veins. Among fossil fern fronds 



1 A good example is figured by Newberry (88) PI. xxv. as a decorticated 

 coniferous stem of Triassio age. 



2 Potoni6 (87). 



