FILICES. 135 



but little with them. Such an account of them moreover would be foreign to 

 the plan of this work, since it would offer very little that is of interest from 

 the botanical point of view ; moreover, our living material supplies us with 

 a much greater variety of forms of nerve-distribution. But that my work 

 may not seem too unconnected and fragmentary, I introduce here a very brief 

 account of the main types of nervation accompanied with a few examples. 



Nervatio Pecopteridis is characterised, as is well known, by pinnately 

 arranged tertiary nerves, which arise at a rather broad angle from the 

 secondary nerves, and, remaining simple or bifurcating, run straight and free 

 to the margin. Nervatio Sphenopteridis is closely allied to it, being 

 essentially distinguished only by the very acute angle which the tertiary 

 nerves form with the secondary. These two types of nervation are not 

 sharply separated from one another ; the form which is as nearly as possible 

 intermediate between them is named by Mettenius Nervatio Eupteridis, 

 and the genus Alethopteris may be quoted as an example of this type 

 among fossil Ferns, though it is actually distinguished from Pecopteris 

 more by habit than by very marked characters. In many cases the incisions 

 in the leaf follow the course of the nervation with great exactness, so that 

 the extreme point of the leaf is only traversed by a median nerve, and this 

 produces Mettenius' Nervatio Caenopteridis. But all fern-leaves which 

 have this character are as a matter of fact reckoned with Sphenopteris, 

 though some of them may on closer examination be found to resemble 

 Pecopteris rather than Sphenopteris as regards the angle of emergence of 

 the nerves. This may be the case, for example, with Sphenopteris Hoen- 

 inghausii, Brongn., to judge by Schimper's figure 1 . Pecopteridae and 

 Sphenopteridae are found in abundance throughout the whole series of 

 Palaeozoic and Mesozoic formations. In the Cainozoic deposits the Ferns 

 are on the whole in a minority as compared with other plants. They pre- 

 dominate in the Coal-measures especially, and by far the greater number of 

 large and highly compound leaves belong to that formation. The oldest 

 known Ferns from the Upper Devonian beds and the Culm are for the most 

 part, though not entirely, Sphenopteridae with unusually complete division 

 of the lamina of the leaf, which appears to be reduced everywhere to a 

 narrow margin accompanying the nerves. Such forms are usually described 

 in the literature as Hymenophyllites, Todea or Rhodea ; as examples may 

 be mentioned Rhodea patentissima, Ett. 2 from the Culm, Todea Lipoldi 3 

 from the roofing-slates of Moravia, Sphenopteris Condrusorum 4 (Psilo- 

 phyton, Crep. 5 ) from the Devonian formation. 



Nervatio Taeniopteridis is more sharply defined. Here the tertiary 

 nerves emerge almost at a right angle and run in a straight line to the 



Schimper (1), t. 29. 2 Schimper (2), p. 108. s Stur (6), t. u, f. 8. 



* Gilkinet (1)., 5 Crepin ,1). 



