138 



HISTORICAL PALEONTOLOGY. 



stems and branches seem to have attained a height of two or 

 three feet ; and they sprang from prostrate " root-stocks " or 



creeping stems. Upon the whole, 

 Principal Davvson is disposed to 

 regard Psilophyton as a "general- 

 ised type" of plants intermediate 

 between the Ferns and the Club- 

 mosses. Lastly, the Devonian de- 

 posits have yielded the remains of 

 the first actual trees with which we 

 are as yet acquainted. About the 

 nature of some of these (Ormoxylon 

 and Dadoxylon) no doubt can be 

 entertained, since their trunks not 

 only show the concentric rings of 

 growth characteristic of exogen- 

 ous trees in general, but their 

 woody tissue exhibits under the 

 microscope the " discs " which are 

 characteristic of the wood of the 

 Pines and Firs (see fig. 2). The 

 singular genus Prototaxites, how- 

 ever, which occurs in an older por- 

 tion of the Devonian series than 

 the above, is not in an absolutely 

 unchallenged position. By Prin- 

 cipal Dawson it is regarded as the 

 trunk of an ancient Conifer the 

 most ancient known ; but Mr 

 Carruthers regards it as more pro- 

 bably the stem of a gigantic sea- 

 weed. The trunks of Prototaxites 

 (fig. 78, A) vary from one to three 

 feet in diameter, and exhibit con- 

 centric rings of growth ; but its 

 woody fibres have not hitherto 

 been clearly demonstrated to pos- 

 sess clipcs. Before leaving the 



Devonian vegetation, it may be mentioned that the hornstone 

 or chert so abundant in the Corniferous limestone of North 

 America has been shown to contain the remains of various 

 microscopic plants {Diatoms and Desmids). We find also in 

 the same siliceous material the singular spherical bodies, with 

 radiating spines, which occur so abundantly in the chalk flints, 

 and which are termed Xanthidia. These may be regarded 



*^g- 77 Restoration of Psilo- 

 phyton firinceps Devonian, Can 

 .fter Da' 



ada. (Afte 



iwson.) 



