LITTLE-KNOWN FERNS. 105 
separated from one another by partition walls. In Horse- 
tails and Calamitez 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 Asterophyllitez, the star-shaped whorls of leaves were 
more strongly developed than in the two other orders. 
There exist, at present, of the Calamariz 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 great 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 (Rhizorcarpeze, or Hydropteridee), 
is least known to us. In their structure these ferns, which 
live 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 pill ferns (Pilularia) of our fresh waters; further, the 
large Azolla which floats in tropical ponds. Most of the 
aquatic ferns are of a delicate nature, and hence ill-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 
