200 Prof. W. C. Williamson and Dr. D. H. Scott. [June 13, 



D. Habit and Dimensions of the Plant. 



In none of our authentic specimens is the stem more than 4 cm. in 

 thickness. Certain cortical impressions, belonging to much larger 

 stems, have been referred to Lyginodendron, but on inconclusive 

 grounds. 



There is one large specimen showing structure, in which only the 

 secondary wood and portions of the pith are preserved. So much of 

 the structure as remains agrees closely with that of Lyginodendron. 

 This specimen may have reached a diameter of 30 cm. or 40 cm., and 

 establishes a certain probability that L. Oldhamium, or some allied 

 species, may have attained the dimensions of a small tree. 



The ordinary specimens must have had upright stems of consider- 

 able height, bearing spirally-arranged, compound, fern-like leaves, 

 separated by internodes about an inch long. The lower parts of the 

 stem gave off on all sides numerous adventitious roots. 



The entire absence of fructification is remarkable, considering the 

 great frequency and admirable preservation of our fossil. It may be 

 explained, either on the hypothesis that the leaves bore very caducous, 

 fern-like sporangia, or by supposing that our material consists entirely 

 of immature specimens. 



II. HETEEANGIUM. 

 Introduction. 



The genus Heterangium differs conspicuously from Lyginodendron, 

 in the structure of the stele of the stem, which in Heterangium, con- 

 tains no pith, but has a solid axis of primary wood. In most other 

 respects the two genera much resemble each other. 



i. Heterangium, Grievii, Will.* 



The original specimens of this species were derived from the 

 Burntisland deposits. At a later date, specimens were found in the 

 coal-measures of Dulesgate, Lancashire, which have been referred to 

 the same species, though they show some slight differences from the 

 original form. 



A. The Stem. 



1. General Structure. The whole interior of the stele is occupied 

 by the primary wood, consisting of tracheides intermixed with con- 

 junctive parenchyma. In most specimens a certain amount of 

 secondary wood has been formed around the central mass. Outside 



* Williamson, ' British Association Reports,' 1871 ; " Organisation," Part IV, 

 1872 ; Part XVII, 1890. 



