172 THE GEOLOGICAL HISTORY OF PLANTS. 



The external appearances of trunks of tree-ferns have been al- 

 ready referred to. 



With respect to tree ferns, the oldest known examples are those 

 from the Middle Devonian of New York and Ohio, which I have de- 

 scribed in the "Journal of the Geological Society," 1871 and 1881. 

 As these are of some interest, I have reproduced their descriptions 

 in a note appended to Chapter III, which see. 



The other forms most frequently occurring in the Carboniferous 

 are Caulopteris, Paloeopteris, and MegapJiyton* Stems showing 

 merely masses of aerial roots are known by the name Psaronius. 



With reference to the classification of Paleozoic ferns, this has 

 hitherto been quite arbitrary, being based on mere form and vena- 

 tion of fronds, but much advance has recently been made in the 

 knowledge of their fructification, warranting a more definite at- 

 tempt at classification. The following are provisional genera usu- 

 ally adopted : 



1. Cyclopteris, Brongniart. Leaflets more or less rounded or 

 wedge-shaped, without midrib, the nerves spreading from the point 

 of attachment. This group includes a great variety of fronds evi- 

 dently of different genera, were their fructification known ; and some 

 of them probably portions of fronds, the other parts of which may 

 be in the next genus. 



2. Neuropteris, Brongniart. Fronds pinnate, and with the 

 leaflets narrowed at the base ; midrib often not distinct, and disap- 

 pearing toward the apex. Nervures equal, and rising at an acute 

 angle. Ferns of this type are among the most abundant in the coal- 

 formation. 



3. Odontopteris, Brongniart. In these the frond is pinnate, and 

 the leaflets are attached by their whole base, with the nerves either 

 proceeding wholly from the base, or in part from an indistinct mid- 

 rib, which soon divides into nervures. 



4. Dictyopteris, Gutbier. This is a beautiful style of fern, with 

 leaflets resembling those of Neuropteris, but the veins arranged in a 

 network of oval spaces. Only a few species are known in the coal- 

 formation. 



5. Lonchopteris, Brongniart. Ferns with netted veins like the 

 above, but with a distinct midrib, and the leaflets attached by the 

 whole base. Of this, also, we can boast but few species. 



6. Sphenopteris, Brongniart. These are elegant ferns, very nu- 

 merous in species, and most difficult to discriminate. Their most 



* See my " Acadian Geology," also below. 



