256 
The Ohio Nnturalist. 
[Vol. V, No. 3, 
a stage or step taken in several independent lines and by all the 
higher groups, it seems best to put them together in a sub-kingdom 
of their own. 
The relationship of some of the included fossil forms is still 
imperfectly understood on account of the absence of properly 
preserved sporangia. The following orders are usually recog- 
nized ; 
1, Calamariales ; 2, Sjdienophyllales ; 3, Salviniales; 4, Mar- 
sileales; 5, Isoetales; (i. Lepidophytales; 7, Selaginellales. The 
first, second, and sixth orders named above are entirely fossil. 
These seven orders fall naturally into five classes which may be 
designated as follows: Calamariae, Sphenophylleae, Hvdropterides 
or water ferns including the Salvinial£s and Marsileales. Isoeteae 
or quillworts and Selaginelleae, including the Selaginellales and 
the fossil Lepidodendrids Sigillarids and other genera belonging 
to the sixth order. 
The Hydropterides are the only plants among the Heterospor- 
ous Pteridophytes that are leptosporangiate. In this respect 
they are similar to some of the homosporous ferns. The resem- 
blance, however, does not extend much farther and it is not prob- 
able that they are a l)ranch from the homosporous leptosporangi- 
ate ferns. The fossil record indicates that the water ferns are 
much the older group. It might be stated that the term lep- 
tosporangiate refers to the origin of the sporangium which in 
these plants originates from an epidermal cell instead of from 
hypodermal tissue as in all other higher plants. The Selaginellas 
are probably descended from the primitive Lycopods but the 
quillworts show no evident relationship to any known homospor- 
ous forms. 
Heterosporous Pteridophytes appear in the first known land 
flora, but these forms were not primitive types; for the primitive 
floras have either not been discovered or else have left no fossil 
trace of their existence. There is some evidence that members of 
this sub-kingdom were present in the Ordovician period ; but 
however that may be they are definitely found in the Silurian and 
became very important in the Devonian. They culminated in 
the Carboniferous, which from a botanical point of view might be 
called the age of Heterosporous Pteridophytes. The coal 
swamps were full of great tree forms belonging to the genera, 
Lepidodendron, Sigillaria, Calamites. and others. These plants 
formed the larger part of the material preserved as coal and were 
therefore of great economic importance for the future welfare of 
man. They declined during the Permian and very few appear to 
have survived the great disturbance known in American geology 
as the Appalachian revolution. These plants were essentially 
moisture-loving and when the great changes occurred which mark 
the transition from the Paleozoic to the Mesozoic the}' seem to 
