LITTLE-KNOWN FERNS. IO5 



separated from one another by partition walls. In Horse- 

 tails and Calamitese the surface is traversed by longitudinal 

 ribs running parallel, as in the case of a fluted column, and 

 the outer skin contains so much silicious earth in the living 

 forms, that it is used for cleansing and polishing. In 

 the AsterophyUitese, the star-shaped whorls of leaves were 

 more strongly developed than in the two other orders. 

 There exist, at present, of the Calamarise only the in- 

 significant Horse-tails (Equisetum), which grow in marshes 

 and on moors; but during the whole of the primary 

 and secondary periods they were represented by gi'eat trees 

 of the genus Equisetites. There existed, at the same time, 

 the closely related order of the Giant Reeds (Calamites), 

 whose strong trunks grew to a height of about fifty feet. 

 The order of the Asterophyllites, on the other hand, con- 

 tained smaller and prettier plants, of a very peculiar form, 

 and belongs exclusively to the primary period. 



Among all Ferns, the history of the third class, that of 

 the Root, or Aquatic Ferns (Rhizorcarpese, or Hydropteridse), 

 is least known to us. In their structure these ferns, which 

 Kve in fresh water, are on the one hand allied to the frond 

 ferns, and on the other to the scaly ferns, but they are more 

 closely related to the latter. Among them are the but 

 little known moss ferns (Salvinia), clover ferns (Marsilea), 

 and piU ferns (Pilularia) of our fresh waters ; further, the 

 large AzoUa which floats in tropical ponds. Most of the 

 aquatic ferns are of a delicate nature, and hence iU-suited 

 for being petrified. This is probably the reason of their 

 fossil remains being so scarce, and of the oldest of those 

 known to us having been found in the Jura system. It is 

 probable, however, that the class is much older, and that it 



