1482 CLASSIFICATION AND SUCCESSION OF PLANTS. 



of these so-called " Fucoids " are wholly destitute of carbon, and 

 are almost certainly not of a vegetable nature. Many of these 

 supposed Sea-weeds — such as those referred to the genera Palceo- 

 chorda and Crossopodia (the Crossochorda of Schimper) — present 

 themselves as simple, often sinuous or convoluted, raised markings 

 on the surface of muddy or arenaceous sediments. Such markings, 

 as specially insisted on by Nathorst, always occur in demi-relief on 

 the under surfaces of the beds in which they are found ; and this 

 fact would strongly support the view that they are really the casts of 

 the trails of marine animals such as Worms or Molluscs, such trails 

 presenting themselves as depressed impressions on the upper sur- 

 faces of the strata. Raised impressions on the under sides of the 

 strata, but branched, are not uncommon, and upon these have been 

 founded such genera as Licrophycus (fig. 1355). These curious 

 fossils may be regarded as probably of the nature of filled-up worm- 

 burrows, rather than as mere surface-trails. 



On the other hand, some of the so-called " Fucoids " of the 

 Ordovician, as also of the Silurian rocks, appear to be truly the 



Fig. 1356. — Buthotrephis gracilis, Hall; a " Fucoid " from the Trenton Limestone 

 (Ordovician) of Ottawa. (Original.) 



remains of marine Algce. This is the case, for example, with some 

 of the fossils which have been referred to the genus Buthotrephis of 

 Hall. The types in question (fig. 1356) present themselves as 

 compressed, branching impressions, sometimes showing leaf-like 

 extremities, and occasionally distinctly carbonaceous. 



