310 ON THE ACE OF THE 



P. Whitbiensis. The main nervo, starting from the midrib near- 

 ly at right angles, bends gi'adually upwards, the nervules diverg- 

 ing slowly from this, or springing from the base near the central 

 nerve, are once and twice furcated, and both they and the central 

 nerve are very delicate. In most of these particulars, it will be 

 seen that our fossil agrees precisely with P. Whitbiensis. The 

 only points of difference seem to be, a somewhat less acute ter- 

 mination of the })innules, their rather greater breadth in propor- 

 tion to their length, and their more delicate nervation. These 

 disagreements are, I presume, too unimportant to separate the 

 Virginia fossil from the P. Whitbiensis, especially when so high 

 an authority as Sternberg has united under this title three of 

 Brongniart's species, presenting much greater diversities, namely, 

 P. Whitbiensis, P. Nebbensis, P. teneris. 



The importance of this fossil, in determining the age of the 

 strata in which it is found, may be infeo-ed from the statement 

 of Erongniart, that " this plant is altogether peculiar- to the Jura 

 formation, and has no analogy with any of those which appertain 

 to the true carboniferous system. 



Pecopteris Muxstekiana. Stenibej'g". 



Of the fossil here referred to, I have met with but one specimen. 

 This consists of a single pinna, with a full an-ay of uncommonly 

 large, and, in general, distinctly marked pinnules. The extraordi- 

 nary size of the pinnules, and their proximity and mode of attach- 

 ment to the rachis, might at first lead us to regard this fossil as 

 identical with the Pecopteris insi^-nis of the Yorkshire Oolite, de- 

 scribed and figured by Lindley and Hutton ; but a closer attention 

 to the plan of nervation, and the form and size of the pinnules of 

 the two, discloses very important points of difference. In the P. 

 insig'nis, eacli nervure, at its junction with the middle nerve, very 

 regularly divides into two branches. In the Virginia fossil, after 

 this forking at the middle nerve, there occurs a further bifurcation 

 of one or of both the branches thus formed. The pinnules of 

 the P. insignis are larger than those of our fossil ; they are, more- 

 over, of a falcated shape, while those of the Virginia species are 



