432 



PAL.^OBOTANY. 



Silurian upwards. Many of the more ancient remains which 

 have been set down as " Fucoids " are certainly of a very 

 dubious and indefinite character. Some of them may be 

 inorganic ; many are doubtless the work of marine w^orms ; 



l,i\Mlf,« 



^'^L., 



1i?8M 



^ip 



Fig. 693. — Lithothumnium ramosissimum, a calcareous Alga, from the " Leitha-Kalk " of 

 the Vienna basin, a, Portion of a mass, of the natural size ; b and c. Transverse and vertical 

 sections of the same magnified 320 diameters. (After Giiiiibel.) 



some possibly belong to land-plants ; but others may be 

 safely set down as veritable sea-weeds. One of these last, 

 from rocks as old as the Lower Silurian, is here figured ; and 

 though it is now so greatly flattened as to be reduced to a 



Fig. 69i.—Bvthotrcphis gracUis, Hall ; a " Fucoid," from the Trenton Limestone 

 of Ottawa. (Original.) 



mere impression, its form and its carbonaceous texture leave 

 no room for doubt as to its vegetable nature (fig. 694). An 

 equally unquestionable plant- — almost certainly a sea-weed 



