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, Sphenophyllales; 3, Salviniales; 4, Mar- 

 sileales; 5, Isoetales; 6. Lepidophytales; 7, Selaginellales. The 

 first, second, and sixth orders named above are entirely fossil. 

 These seven orders fall naturally into five classes which mav be 

 designated as follows: Calamarice, Sphenophyllege, Hydropterides 

 or water ferns including the Salviniales and Marsileales, Isoetege 

 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 branch 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 culmniated 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 they seem to 



