GENERAL SUCCESSION OF PLANTS IN TIME. 1483 



In addition to Alga, the Ordovician rocks have yielded a few 

 unquestionable plant-remains which are regarded by Dawson as 

 probably having been of a higher grade. Thus, the Protannularia 

 of the Arenig rocks of Britain, and the Sphenothallus of the Cin- 

 cinnati group of North America, are provisionally referred to the 

 Rhizocarps ; while the Protostigma of the latter formation is looked 

 upon as possibly allied to the Lycopods. 



In the Silurian rocks we meet with various Sea-weeds of an appa- 

 rently unquestionable character, and along with these are found the 

 remains of plants of a higher type. The most abundant of these 

 belong to the genus Psilophyton, regarded by Dawson as forming a 

 connecting link between the Rhizocarps and the Lycopods. The 

 Glyptodendron of the Clinton beds may perhaps be related to Lepi- 

 dodendron, but the supposed Silurian fern, described under the name 

 of Eopteris, is not truly organic. Lastly, in the Silurian rocks are 

 found the first traces of the singular tree-like plants originally de- 

 scribed by Dawson under the name of Prototaxites, but now termed 

 Nematophyton. The characters of this will be briefly noted imme- 

 diately ; but it may be mentioned here that the curious spore-like 

 bodies, which have been described from the Silurian rocks under 

 the name of Pachytheca, are regarded by Dawson as not improbably 

 belonging to this plant. 



In the Devonian rocks, as more especially shown by Sir William 

 Dawson, we have evidence of an abundant flora, consisting of both 

 aquatic and terrestrial plants. Of the Devonian fossils which have 

 been referred to the Algce, the most singular is the genus Spirophy- 

 ton, comprising certain broad, spirally twisted impressions, which are 

 very abundant in some of the lower beds of the Devonian series of 

 North America. Similar impressions — often spoken of under the 

 name of " Cauda Galli" — occur in the Lower Carboniferous rocks of 

 Europe ; but their real nature is not free from doubt. We may also 

 notice here the highly remarkable tree-like plants for which the 

 genus Nematophyton {Prototaxites) has been proposed by Dawson, 

 since these cannot at present be definitely referred to any recog- 

 nised group of existing plants. According to Dawson, the plants in 

 question (fig. 1357) are "trees of large size, with a coaly bark and 

 large spreading roots, having the surface of the stem smooth or 

 irregularly ribbed, but with a nodose or jointed appearance. In- 

 ternally, they show a tissue of long, cylindrical tubes, traversed by a 

 complex network of horizontal tubes, thinner-walled and of smaller 

 size." The tubes appear to be arranged in concentric zones, but 

 the plant was not truly exogenous, and it is doubtful if a genuine 

 bark was present. The systematic place of Nematophyton is un- 

 certain, but the balance of evidence would seem to be in favour of 

 its reference to the Thallophytes, and it is regarded by Dawson as 



