274 



HISTORICAL GEOLOGY. 



seas seem to have swarmed with a life as abundant as any 

 now existing, but wholly different in species, in genera, 

 and even in families, not only from any now living, but 

 from those living in any other geological period. About 

 20,000 species are described from the Paleozoic, and of 

 these at last one half, i. e., 10,000 species, are from the 

 Silurian ; and of course these are but a very small frac- 

 tion of the number which actually existed. The number 

 being so great, and the forms so unfamiliar to the pupil, 

 it is impossible to do more than mention and figure a few 

 of the most common and striking forms. 



Plants. 



The only kind of plants which are found so early are' 

 allied to sea-weeds.* As it is very difficult to determine 

 these species from the very imperfect impressions of them 

 left in the rocks, we shall call them by the general name 



FIG. 170. 



FIG. 171. 



FIGS. 170, 171. Silurian plants : 170. Sphenothallus angustifolius. in. Buthotre- 

 phis gracilis. 



of Fucoids, i. e., fucus-like plants, from their general 

 resemblance to Fucus (tangle or kelp). We give a few 



* A few small vascular cryptogams, allied to club-mosses, have 

 been recently found in the Silurian. 



