226 STUDIES IN FOSSIL BOTANY 



by Williamson 1 in 1871, but except as proving the 

 presence of a secondary zone of wood, and of a peri- 

 derm, the investigation yielded little result. The 

 same author described at the same time, under the 

 name of Diploxylon, a vascular cylinder which he 

 afterwards referred to Sigillaria reniformis, one of the 

 Rhytidolepis group {I.e. Figs. 33 and 34). In this 

 stem there is a perfectly continuous zone of primary 

 wood, with a crenulated outer margin, beyond which is 

 a much broader layer of secondary xylem. The first 

 really good English example of a stem of the Rhytido- 

 lepis type, with structure preserved, was a specimen 

 which came several years ago into the hands of Professor 

 Boyd Dawkins, and has not yet been fully described. 2 

 The specimen is a fragment only, forming a segment 

 of the stem, but it includes the whole thickness of the 

 tissues from the pith to the outer surface (Fig. 93). The 

 latter is ribbed in the characteristic manner of Rhytido- 

 lepis, and there appears to be no doubt of its belonging 

 to a Sigillaria of that type. From the inner margin 

 of the wood to the exterior surface of the ribs the 

 radius of the specimen is about 1 8 mm. The zone of 

 primary xylem, which is less than a millimetre in thick- 

 ness, is quite continuous as far as it can be traced. Its 

 outer edge, where the smallest elements are placed, is 

 crenulated, just as in Williamson's specimen, referred 

 to above. The secondary zone of wood, which has 

 the usual structure, is about 4 mm. thick, and has an 

 undulating outer surface, corresponding to the crenula- 



1 " Organisation of Fossil Plants of Coal-measures," Part ii. 1872. 



2 For photographs of this specimen, one of which is reproduced in Fig. 

 93, I am indebted to Mr. A. Gepp of the British Museum, Natural History 

 Department. 



