2 Weiss, A Tylodendron-Uke Fossil. 



a genus instituted in 1869 by Weiss/ who considered it 

 to be a conifer, the slitlike depression being regarded by 

 him as a resin-canal, such as occur in most of our recent 

 conifers. Potonie," however, came to the conclusion that 

 Tylodendron is not the complete tissue of a fossil plant, 

 but only the pith-cast of a coniferous tree, possibly 

 closely related to Araucaria. The areolae he correctly 

 identified with the markings of the primary medullary 

 rays where they join the pith, while the depressed grooves 

 around them indicate the positions of the primary wood. 

 The narrow slit in the cushion-like area represents the 

 position of the leaf-trace not quite median to the cushion, 

 but slightly on one side. 



Potonie has shown by the comparison of 7'ylodendron 

 with the pith cast of Araucaria brasiliana that the 

 marking of the two is in close agreement, and also that 

 the occasional barrel-like swellings appear to represent 

 the increase in thickness of the medullary region at the 

 points where the whorls of branches are given off. In 

 one specimen of Tylodendron, at all events, described 

 by Potonie, branch scars and remains of branches have 

 been met with, and also portions of the surrounding 

 woody cylinder. P'or the most part, however, the speci- 

 mens of Tylodendron are mere pith casts. In some 

 cases, however, the pith has been silicified, so that its 

 structure and that of the adjacent woody tissues have 

 been able to be examined by Potonie. These specimens 

 have shown that the wood possessed the structure 

 generally described for Araucarioxylon, and particularly 

 with the species A. Rhodeanuin. 



> Weiss, Ch. E. " Fossile Flora der jiingsten Steinkohlen-foniiation 

 und der Rothliegenden im Saar-Rhein-Gebiet." Bonn, 1869. 



2 Potonie, H. " Uber die fossile Pflanzengattung Tylodendron," 

 /ahrbuch der Konigl. preitss. Geologischen Laudcsaiistalt, 1S87. 



